allow boards to customize compiler options on a per-file/dir basis

With our Blackfin boards, we like to build the compression routines with
-O2 as our tests show a pretty good size/speed tradeoff.  For the rest of
U-Boot though, we want to stick with the default -Os as that is mostly
control code.  So in our case, we would add a line like so to the board
specific config.mk file:
	CFLAGS_lib_generic += -O2

Now all files under lib_generic/ will have -O2 appended to their build.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>