As in bsd.lib.mk and bsd.prog.mk, we need to remap paths in the kernel
and in kernel modules when reproducible builds are configured. Add
directives to kern.post.mk and kmod.mk to remap the src, obj and sysroot
dirs.

For the sysroot, we need to pass the value of the SYSROOT variable to
the kernel makefiles, which involves changing Makefile.inc1. In so
doing I reverted a portion of commit 35fb812adfc21; I can't see any harm
in passing the SYSROOT variable down.