kbuild: enable use of AFLAGS and CFLAGS on commandline

The previous patches was preparation.
With this patch we can now say:
make CFLAGS=-Os vmlinux

And the option specified will be appended to the
options passed to gcc for C files.

For assembler use:
make AFLAGS=-foo vmlinux
for the same functionality.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

diff --git a/Makefile b/Makefile
index 8d3f664..ad0add0 100644
--- a/Makefile
+++ b/Makefile
@@ -523,6 +523,10 @@
 # disable pointer signed / unsigned warnings in gcc 4.0
 KBUILD_CFLAGS += $(call cc-option,-Wno-pointer-sign,)
 
+# Add user supplied AFLAGS and CFLAGS as the last assignments
+KBUILD_AFLAGS += $(AFLAGS)
+KBUILD_CFLAGS += $(CFLAGS)
+
 # Use --build-id when available.
 LDFLAGS_BUILD_ID = $(patsubst -Wl$(comma)%,%,\
 			      $(call ld-option, -Wl$(comma)--build-id,))