At present, the PREREQ-variable is effectively only used to detect if prerequiste packages haven't run "make all" before make is run in a given package. Also, it's only useful in $(TOPDIR). This commit splits the variable up into PREREQ_BUILD and PREREQ_RUN, and makes the variables available in every Makefile of a package by placing them in defs.mk instead of topdir.mk.
This also fixes a problem that PREREQ was cached before being filled, hence empty. Which effectively wasn't much of a problem, because it was basically unused, but still.
Signed-off-by: Jan Lindemann <jan@janware.com>