This patch introduces the $(ALL_CFLAGS) variable, which holds
$(BASIC_CFLAGS) as well as userdefined $(CFLAGS) and then
consistently uses that variable where both were used anyway.

Since we're in the area, we optimize the sparse running a
bit, getting rid of the shell and just letting sparse iterate
over the files.

Signed-off-by: Andreas Ericsson <ae@op5.se>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>