Make App.proj_dir() return an absolute path. This looks like a good
idea, because some of the $(call proj_query xxx) paths end up being
relative, because they get proj_dir()'s idea of a directory
prepended. This prohibits caching them in $(TOPDIR)/make/.cache.mk
for make benefit glorious nation of performance.
Signed-off-by: Jan Lindemann <jan@janware.com>