Index: Makefile
===================================================================
--- Makefile
+++ Makefile
@@ -5,10 +5,12 @@
 #
 # universe            - *Really* build *everything* (buildworld and
 #                       all kernels on all architectures).  Define
-#                       MAKE_JUST_KERNELS to only build kernels,
-#                       MAKE_JUST_WORLDS to only build userland.
+#                       MAKE_JUST_KERNELS or WITHOUT_WORLDS to only build kernels,
+#                       MAKE_JUST_WORLDS or WITHOUT_KERNELS to only build userland.
 # tinderbox           - Same as universe, but presents a list of failed build
 #                       targets and exits with an error if there were any.
+# worlds	      - Same as universe, except just makes the worlds.
+# kernels	      - Same as universe, except just makes the kernels.
 # buildworld          - Rebuild *everything*, including glue to help do
 #                       upgrades.
 # installworld        - Install everything built by "buildworld".
@@ -104,6 +106,15 @@
 # For more information, see the build(7) manual page.
 #
 
+.if defined(UNIVERSE_TARGET) || defined(MAKE_JUST_WORLDS) || defined(WITHOUT_KERNELS)
+__DO_KERNELS=no
+.endif
+.if defined(MAKE_JUST_KERNELS) || defined(WITHOUT_WORLDS)
+__DO_WORLDS=no
+.endif
+__DO_WORLDS?=yes
+__DO_KERNELS?=yes
+
 # This is included so CC is set to ccache for -V, and COMPILER_TYPE/VERSION
 # can be cached for sub-makes. We can't do this while still running on the
 # old fmake from FreeBSD 9.x or older, so avoid including it then to avoid
@@ -468,7 +479,7 @@
 	@cd ${.CURDIR}; ${SUB_MAKE} UNIVERSE_TARGET=kernel-toolchain universe
 
 kernels: .PHONY
-	@cd ${.CURDIR}; ${SUB_MAKE} UNIVERSE_TARGET=buildkernel universe
+	@cd ${.CURDIR}; ${SUB_MAKE} universe -DWITHOUT_WORLDS
 
 worlds: .PHONY
 	@cd ${.CURDIR}; ${SUB_MAKE} UNIVERSE_TARGET=buildworld universe
@@ -483,19 +494,20 @@
 .if make(universe) || make(universe_kernels) || make(tinderbox) || \
     make(targets) || make(universe-toolchain)
 #
-# Don't build rarely used architectures unless requested.
+# Don't build rarely used, semi-supported architectures unless requested.
 #
 .if defined(EXTRA_TARGETS)
 EXTRA_ARCHES_mips=	mipsel mipshf mipselhf mips64el mips64hf mips64elhf
 EXTRA_ARCHES_mips+=	mipsn32
+# powerpcspe excluded from main list until clang fixed
+EXTRA_ARCHES_powerpc=	powerpcspe
 .endif
 TARGETS?=amd64 arm arm64 i386 mips powerpc riscv
 _UNIVERSE_TARGETS=	${TARGETS}
 TARGET_ARCHES_arm?=	armv6 armv7
 TARGET_ARCHES_arm64?=	aarch64
 TARGET_ARCHES_mips?=	mips mips64 ${EXTRA_ARCHES_mips}
-# powerpcspe excluded until clang fixed
-TARGET_ARCHES_powerpc?=	powerpc powerpc64
+TARGET_ARCHES_powerpc?=	powerpc powerpc64 ${EXTRA_ARCHES_powerpc}
 TARGET_ARCHES_riscv?=	riscv64 riscv64sf
 .for target in ${TARGETS}
 TARGET_ARCHES_${target}?= ${target}
@@ -521,11 +533,7 @@
 .endif
 .endfor
 
-.if defined(UNIVERSE_TARGET)
-MAKE_JUST_WORLDS=	YES
-.else
 UNIVERSE_TARGET?=	buildworld
-.endif
 KERNSRCDIR?=		${.CURDIR}/sys
 
 targets:	.PHONY
@@ -634,7 +642,7 @@
 .endfor
 .endif	# !make(targets)
 
-.if !defined(MAKE_JUST_KERNELS)
+.if ${__DO_WORLDS} == "yes"
 universe_${target}_done: universe_${target}_worlds .PHONY
 .for target_arch in ${TARGET_ARCHES_${target}}
 universe_${target}_worlds: universe_${target}_${target_arch} .PHONY
@@ -658,9 +666,9 @@
 	    ${MAKEFAIL}))
 	@echo ">> ${target}.${target_arch} ${UNIVERSE_TARGET} completed on `LC_ALL=C date`"
 .endfor
-.endif # !MAKE_JUST_KERNELS
+.endif # ${__DO_WORLDS} == "yes"
 
-.if !defined(MAKE_JUST_WORLDS)
+.if ${__DO_KERNELS} == "yes"
 universe_${target}_done: universe_${target}_kernels .PHONY
 universe_${target}_kernels: universe_${target}_worlds .PHONY
 universe_${target}_kernels: universe_${target}_prologue .MAKE .PHONY
@@ -673,7 +681,7 @@
 	fi
 	@cd ${.CURDIR}; ${SUB_MAKE} ${.MAKEFLAGS} TARGET=${target} \
 	    universe_kernels
-.endif # !MAKE_JUST_WORLDS
+.endif # ${__DO_KERNELS} == "yes"
 
 # Tell the user the worlds and kernels have completed
 universe_${target}: universe_${target}_done
Index: share/man/man7/build.7
===================================================================
--- share/man/man7/build.7
+++ share/man/man7/build.7
@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd December 3, 2018
+.Dd March 29, 2020
 .Dt BUILD 7
 .Os
 .Sh NAME
@@ -289,6 +289,18 @@
 including
 .Pa LINT .
 This command takes a long time.
+.It Cm kernels
+Like
+.Cm universe
+with
+.Va WITHOUT_WORLDS
+defined so only the kernels for each architecture are built.
+.It Cm worlds
+Like
+.Cm universe
+with
+.Va WITHOUT_KERNELS
+defined so only the worlds for each architecture are built.
 .It Cm update
 Get updated sources as configured in
 .Xr make.conf 5 .
@@ -761,7 +773,7 @@
 .Pp
 Builds using the
 .Cm universe
-target are influenced by the following
+and related targets are influenced by the following
 .Xr make 1
 variables:
 .Bl -tag -width ".Va MAKE_JUST_KERNELS"
@@ -775,11 +787,26 @@
 Only build kernels for each supported architecture.
 .It Va MAKE_JUST_WORLDS
 Only build worlds for each supported architecture.
+.It Va WITHOUT_WORLDS
+Only build kernels for each supported architecture.
+.It Va WITHOUT_KERNELS
+Only build worlds for each supported architecture.
 .It Va UNIVERSE_TARGET
 Execute the specified
 .Xr make 1
 target for each supported architecture instead of the default action of
 building a world and one or more kernels.
+This variable implies
+.Va WITHOUT_KERNELS .
+.It Va TARGETS
+Only build the listed targets instead of each supported architecture.
+.It Va EXTRA_TARGETS
+In addition to the supported architectures, build the semi-supported
+architectures.
+A semi-supported architecture has build support in the
+.Fx
+tree, but receives significantly less testing and is generally for
+fringe uses that do not have a wide appeal.
 .El
 .Sh FILES
 .Bl -tag -width ".Pa /usr/share/examples/etc/make.conf" -compact