r315170 (by imp):
Adopt SRCTOP in usr.bin
Prefer ${SRCTOP}/foo over ${.CURDIR}/../../foo and ${SRCTOP}/usr.bin/foo
over ${.CURDIR}/../foo for paths in Makefiles.
Silence on: arch@ (twice)
Off by default, build behaves normally.
WITH_META_MODE we get auto objdir creation, the ability to
start build from anywhere in the tree.
Still need to add real targets under targets/ to build packages.
Differential Revision: D2796
Reviewed by: brooks imp
