Take advantage of new bmake feature to only consider Makefile.depend
as invalidating DIRDEPS_CACHE.
When bootstrapping allow more filtering via .MAKE.DEPENDFILE_BOOTSTRAP_SED
Move some comments back to where they make sense.
meta.sys.mk: add META_COOKIE_TOUCH and META_NOPHONY to better handle some
targets in meta mode vs non-meta mode.
Also use .MAKE.META.IGNORE_PATHS to ignore mtime of makefiles - which do
not matter in meta mode.
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
