This commit tries to remove the necessity to call projects.py from $(TOPDIR) to
speed up recursive builds over all projects yet again. This is a major
undertaking. There are two variables which are filled py projects.py in
$(TOPDIR): PREREQ and PREREQ_DIRS. Sadly, the latter is a path relative to
$(TOPDIR)/make, so this is kind of pointless. Unless the cache is maintained in
$(TOPDIR), a thing I tried to avoid. So this commit is only able to cache
$(PREREQ), not $(PREREQ_DIRS), which still is a hassle. Introduced defs-dirs.mk
for that, to make it accessible to make.mk, and modified all the other parts of
the machinery, too.
Signed-off-by: Jan Lindemann <jan@janware.com>
