bootstrap-packages handle optional DIRDEPS

Add logic to bootstrap-packages.sh to handle constructs like:

SUBDIR.${MK_OPT}+= sub

and:

.if ${MK_OPT} == "yes"
SUBDIR+= sub
.endif

there are other methods used in various makefiles
so this will be better than nothing, but not complete coverage.

In either case a reldir will be added to
targets/packages/$package/Makefile.depend.options
rather than
targets/packages/$package/Makefile.depend

It does not matter that in many cases the Makefile.depend will end up
with no DIRDEPS.

Also check that parent Makefile does not have a subdir commented out,
before including it.

If BOOTSTRAP_PACKAGES_FLAGS is -v we report each Makefile.depend[.options]
we create.

Reviewed by: stevek