Rework r254951 to not reach into private OBJDIR areas.

The original change was dealing with the build wanting to run a newer
install(1) that was not yet installed.  The solution to look into the private
legacy directory of the existing build conflicts with 2 upcoming features: a
changed OBJDIR format, and splitting the host tools into arch-dependent and
arch-independent directories.  Rather than hardcoding and changing the paths in
this script, just let kernel-toolchain do the work, while disabling much of the
meat.  With -j15 this finishes in 25 seconds for me and 117 seconds with -j1.
All that is really needed is bootstrap-tools, but the system is not currently
written in a way that all previous dependent steps will have ran.  The previous
steps, such as _worldtmp, are being reworked and renamed and so cannot be
relied upon to be right.

Sponsored by:	Dell EMC Isilon

release/scripts/mm-mtree.sh

@@ -81,11 +81,6 @@ if [ ! -f ${SOURCEDIR}/Makefile.inc1 -a \
 fi
 
 # Setup make to use system files from SOURCEDIR
-objp=${MAKEOBJDIRPREFIX}
-[ -z "${objp}" ] && objp=/usr/obj
-legacydir=${objp}${SOURCEDIR}/tmp/legacy
-legacypath=${legacydir}/usr/sbin:${legacydir}/usr/bin:${legacydir}/bin
-MM_MAKE_ARGS="${MM_MAKE_ARGS} PATH=${legacypath}:${PATH}"
 MM_MAKE="make ${ARCHSTRING} ${MM_MAKE_ARGS} -m ${SOURCEDIR}/share/mk"
 
 delete_temproot () {
@@ -121,6 +116,9 @@ echo ''
   esac
   od=${TEMPROOT}/usr/obj
   ${MM_MAKE} DESTDIR=${TEMPROOT} distrib-dirs &&
+  MAKEOBJDIRPREFIX=$od ${MM_MAKE} kernel-toolchain \
+      MK_TOOLCHAIN=no MK_CROSS_COMPILER=no \
+      MK_CDDL=no MK_TESTS=no MK_RESCUE=no &&
   MAKEOBJDIRPREFIX=$od ${MM_MAKE} _obj SUBDIR_OVERRIDE=etc &&
   MAKEOBJDIRPREFIX=$od ${MM_MAKE} everything SUBDIR_OVERRIDE=etc &&
   MAKEOBJDIRPREFIX=$od ${MM_MAKE} DESTDIR=${TEMPROOT} distribution;} ||