Prefer ${SRCTOP}/foo over ${.CURDIR}/../../foo and ${SRCTOP}/usr.bin/foo
over ${.CURDIR}/../foo for paths in Makefiles.
Differential Revision: https://reviews.freebsd.org/D9932
Sponsored by: Netflix
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
