Makefile: fix parallel build

During parallel build, the top Makefile spawns multiple sub-makes for
targets in cpu/$(CPU) and $(dir $(LDSCRIPT)). If the .depend files are
not present in these directories, the sub-makes may end up generating
these files simultaneously which leads to corrupted content.

A typical error message is:

.depend:39: *** multiple target patterns.  Stop.

This patch serializes the creation of .depend in cpu/$(CPU) and
$(dir $(LDSCRIPT)) by adding these directories to the depend target
in the top Makefile.

Other directories in $(LIBS) are not affected since they contain only
one Make target and thus only one sub-make is spawned per directory.

Signed-off-by: Daniel Hobi <daniel.hobi@schmid-telecom.ch>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>