If MAKEOBJDIRPREFIX is set, use it for default OBJROOT.
If MAKEOBJDIR is empty or not a suitable value (no '/')
set a default that works.
Reviewed by: bdrewery
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
By moving META_MODE bits from local.sys.mk, they are easier
to skip when MK_META_MODE=no
Update some filters to cope with sync from head.
If buildworld etc or WITHOUT_META_MODE disable all the META_MODE
related options.
