Index: sys/conf/dtb.build.mk
===================================================================
--- /dev/null
+++ sys/conf/dtb.build.mk
@@ -0,0 +1,95 @@
+# $FreeBSD$
+
+.include <bsd.init.mk>
+# Grab all the options for a kernel build. For backwards compat, we need to
+# do this after bsd.own.mk.
+.include "kern.opts.mk"
+
+DTC?=		dtc
+
+.if !defined(SYSDIR)
+.if defined(S)
+SYSDIR=	${S}
+.else
+# Search for kernel source tree in standard places.
+.for _dir in ${.CURDIR}/../.. ${.CURDIR}/../../.. /sys /usr/src/sys
+.if exists(${_dir}/kern/)
+SYSDIR=	${_dir:tA}
+.endif
+.endfor
+.endif	# defined(S)
+.endif	# defined(SYSDIR)
+
+.if !defined(SYSDIR) || !exists(${SYSDIR}/kern/)
+.error "can't find kernel source tree"
+.endif
+
+.PATH: ${SYSDIR}/gnu/dts/${MACHINE} ${SYSDIR}/dts/${MACHINE} ${SYSDIR}/dts/${MACHINE}/overlays
+
+.if defined(DTS)
+.for _dts in ${DTS}
+_dtb=${_dts:T:R:S/$/.dtb/}
+.if !empty(DTS_${_dtb})
+.warning "Duplicated DTS ${_dts}"
+.endif
+
+DTB:=${DTB} ${_dtb}
+DTS_${_dtb}:=${_dts}
+.endfor
+.endif
+
+.if defined(DTSO)
+.for _dtso in ${DTSO}
+_dtso_path=${_dtso:C/^([^\/])/overlays\/\\1/}
+_dtbo=${_dtso:T:R:S/$/.dtbo/}
+.if !empty(DTSO_${_dtbo})
+.warning "Duplicated DTSO ${_dtso}"
+.endif
+
+DTBO:=${DTBO} ${_dtbo}
+DTSO_${_dtbo}:=${_dtso_path}
+.endfor
+.endif
+
+.if defined(DTB)
+.export DTC ECHO
+.for _dtb in ${DTB}
+${_dtb}:	${DTS_${_dtb}} ${OP_META}
+	@${ECHO} Generating ${.TARGET} from ${DTS_${.TARGET}}
+	@${SYSDIR}/tools/fdt/make_dtb.sh ${SYSDIR} ${DTS_${.TARGET}} ${.OBJDIR}
+.endfor
+.endif
+
+.if defined(DTBO)
+.export DTC ECHO
+.for _dtbo in ${DTBO}
+${_dtbo}:	${DTSO_${_dtbo}} ${OP_META}
+	@${ECHO} Generating ${.TARGET} from ${DTSO_${.TARGET}}
+	@${SYSDIR}/tools/fdt/make_dtbo.sh ${SYSDIR} ${DTSO_${.TARGET}} ${.OBJDIR}
+.endfor
+.endif
+
+_dtbinstall:
+# Need to create this because installkernel doesn't invoke mtree with BSD.root.mtree
+# to make sure the tree is setup properly. We don't recreate it to avoid duplicate
+# entries in the NO_ROOT case.
+	test -d ${DESTDIR}${DTBDIR} || ${INSTALL} -d -o ${DTBOWN} -g ${DTBGRP} ${DESTDIR}${DTBDIR}
+.for _dtb in ${DTB}
+.if ${MACHINE_CPUARCH} == "aarch64"
+	# :H:T here to grab the vendor component of the DTB path in a way that
+	# allows out-of-tree DTS builds, too.  We make the assumption that
+	# out-of-tree DTS will have a similar directory structure to in-tree,
+	# with .dts files appearing in a vendor/ directory.
+	test -d ${DESTDIR}${DTBDIR}/${_dtb:H:T} || ${INSTALL} -d -o ${DTBOWN} -g ${DTBGRP} ${DESTDIR}${DTBDIR}/${_dtb:H:T}
+	${INSTALL} -o ${DTBOWN} -g ${DTBGRP} -m ${DTBMODE} \
+	    ${_INSTALLFLAGS} ${_dtb:T} ${DESTDIR}${DTBDIR}/${_dtb:H:T}
+.else
+	${INSTALL} -o ${DTBOWN} -g ${DTBGRP} -m ${DTBMODE} \
+	    ${_INSTALLFLAGS} ${_dtb} ${DESTDIR}${DTBDIR}/
+.endif
+.endfor
+	test -d ${DESTDIR}${DTBODIR} || ${INSTALL} -d -o ${DTBOWN} -g ${DTBGRP} ${DESTDIR}${DTBODIR}
+.for _dtbo in ${DTBO}
+	${INSTALL} -o ${DTBOWN} -g ${DTBGRP} -m ${DTBMODE} \
+	    ${_INSTALLFLAGS} ${_dtbo} ${DESTDIR}${DTBODIR}/
+.endfor
Index: sys/conf/dtb.mk
===================================================================
--- sys/conf/dtb.mk
+++ sys/conf/dtb.mk
@@ -8,6 +8,8 @@
 #
 # DTS		List of the dts files to build and install.
 #
+# DTSO		List of the dts overlay files to build and install.
+#
 # DTBDIR	Base path for dtb modules [/boot/dtb]
 #
 # DTBOWN	.dtb file owner. [${BINOWN}]
@@ -20,7 +22,7 @@
 #
 # +++ targets +++
 #
-# 	install:
+#	install:
 #               install the kernel module; if the Makefile
 #               does not itself define the target install, the targets
 #               beforeinstall and afterinstall may also be used to cause
@@ -28,78 +30,15 @@
 #		is executed.
 #
 
-.include <bsd.init.mk>
-# Grab all the options for a kernel build. For backwards compat, we need to
-# do this after bsd.own.mk.
-.include "kern.opts.mk"
-
-DTC?=		dtc
-
-# Search for kernel source tree in standard places.
-.for _dir in ${.CURDIR}/../.. ${.CURDIR}/../../.. /sys /usr/src/sys
-.if !defined(SYSDIR) && exists(${_dir}/kern/)
-SYSDIR=	${_dir:tA}
-.endif
-.endfor
-.if !defined(SYSDIR) || !exists(${SYSDIR}/kern/)
-.error "can't find kernel source tree"
-.endif
-
-.SUFFIXES: .dtb .dts .dtbo .dtso
-
-.PATH: ${SYSDIR}/gnu/dts/${MACHINE} ${SYSDIR}/dts/${MACHINE} ${SYSDIR}/dts/${MACHINE}/overlays
-
-DTB=${DTS:R:S/$/.dtb/}
-DTBO=${DTSO:R:S/$/.dtbo/}
+.include "dtb.build.mk"
 
+.if !target(install) && !target(realinstall)
 all: ${DTB} ${DTBO}
-
-.if defined(DTS)
-.export DTC ECHO
-.for _dts in ${DTS}
-${_dts:R:S/$/.dtb/}:	${_dts} ${OP_META}
-	@${ECHO} Generating ${.TARGET} from ${_dts}
-	@${SYSDIR}/tools/fdt/make_dtb.sh ${SYSDIR} ${_dts} ${.OBJDIR}
-CLEANFILES+=${_dts:R:S/$/.dtb/}
-.endfor
-.endif
-
-.if defined(DTSO)
-.export DTC ECHO
-.for _dtso in ${DTSO}
-${_dtso:R:S/$/.dtbo/}:	${_dtso} ${OP_META}
-	@${ECHO} Generating ${.TARGET} from ${_dtso}
-	@${SYSDIR}/tools/fdt/make_dtbo.sh ${SYSDIR} overlays/${_dtso} ${.OBJDIR}
-CLEANFILES+=${_dtso:R:S/$/.dtbo/}
-.endfor
-.endif
-
-.if !target(install)
-.if !target(realinstall)
 realinstall: _dtbinstall
-.ORDER: beforeinstall _kmodinstall
-_dtbinstall:
-# Need to create this because installkernel doesn't invoke mtree with BSD.root.mtree
-# to make sure the tree is setup properly. We don't recreate it to avoid duplicate
-# entries in the NO_ROOT case.
-	test -d ${DESTDIR}${DTBDIR} || ${INSTALL} -d -o ${DTBOWN} -g ${DTBGRP} ${DESTDIR}${DTBDIR}
-.for _dtb in ${DTB}
-.if ${MACHINE_CPUARCH} == "aarch64"
-	test -d ${DESTDIR}${DTBDIR}/${_dtb:H} || ${INSTALL} -d -o ${DTBOWN} -g ${DTBGRP} ${DESTDIR}${DTBDIR}/${_dtb:H}
-	${INSTALL} -o ${DTBOWN} -g ${DTBGRP} -m ${DTBMODE} \
-	    ${_INSTALLFLAGS} ${_dtb:T} ${DESTDIR}${DTBDIR}/${_dtb:H}
-.else
-	${INSTALL} -o ${DTBOWN} -g ${DTBGRP} -m ${DTBMODE} \
-	    ${_INSTALLFLAGS} ${_dtb} ${DESTDIR}${DTBDIR}/
-.endif
-.endfor
-	test -d ${DESTDIR}${DTBODIR} || ${INSTALL} -d -o ${DTBOWN} -g ${DTBGRP} ${DESTDIR}${DTBODIR}
-.for _dtbo in ${DTBO}
-	${INSTALL} -o ${DTBOWN} -g ${DTBGRP} -m ${DTBMODE} \
-	    ${_INSTALLFLAGS} ${_dtbo} ${DESTDIR}${DTBODIR}/
-.endfor
-.endif # !target(realinstall)
-.endif # !target(install)
+.ORDER: beforeinstall _dtbinstall
+
+CLEANFILES+=${DTB} ${DTBO}
+.endif # !target(install) && !target(realinstall)
 
 .include <bsd.dep.mk>
 .include <bsd.obj.mk>
Index: sys/conf/kern.post.mk
===================================================================
--- sys/conf/kern.post.mk
+++ sys/conf/kern.post.mk
@@ -8,6 +8,16 @@
 # should be defined in the kern.pre.mk so that port makefiles can
 # override or augment them.
 
+.if defined(DTS) || defined(DTSO)
+.include "dtb.build.mk"
+
+KERNEL_EXTRA+=	${DTB} ${DTBO}
+CLEAN+=		${DTB} ${DTBO}
+
+kernel-install: _dtbinstall
+.ORDER: beforeinstall _dtbinstall
+.endif
+
 # In case the config had a makeoptions DESTDIR...
 .if defined(DESTDIR)
 MKMODULESENV+=	DESTDIR="${DESTDIR}"