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>