FreeBSD/src bc4d122 (r344242)etc/mtree BSD.root.dist, stand/powerpc/uboot Makefile

powerpc/boot: Move ubldr to /boot/uboot, and make this a separate filesystem

Summary:
Now that mpc85xx can boot via ubldr, move ubldr to a separate
filesystem, mounted on /boot/uboot, so that a fresh install can boot correctly.

Reviewed By: nwhitehorn
Differential Revision: https://reviews.freebsd.org/D18709
DeltaFile
+8-4usr.sbin/bsdinstall/partedit/partedit_powerpc.c
+2-0etc/mtree/BSD.root.dist
+1-0stand/powerpc/uboot/Makefile
+11-43 files

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