kbuild: enable 'make AFLAGS=...' to add additional options to AS

The variable AFLAGS is a wellknown variable and the usage by
kbuild may result in unexpected behaviour.
On top of that several people over time has asked for a way to
pass in additional flags to gcc.

This patch replace use of AFLAGS with KBUILD_AFLAGS all over
the tree.

Patch was tested on following architectures:
alpha, arm, i386, x86_64, mips, sparc, sparc64, ia64, m68k, s390

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/arch/blackfin/Makefile b/arch/blackfin/Makefile
index 52a6ac0..3689337 100644
--- a/arch/blackfin/Makefile
+++ b/arch/blackfin/Makefile
@@ -54,7 +54,7 @@
 rev-$(CONFIG_BF_REV_ANY)  := any
 
 KBUILD_CFLAGS += -mcpu=$(cpu-y)-$(rev-y)
-AFLAGS += -mcpu=$(cpu-y)-$(rev-y)
+KBUILD_AFLAGS += -mcpu=$(cpu-y)-$(rev-y)
 
 head-y   := arch/$(ARCH)/mach-$(MACHINE)/head.o arch/$(ARCH)/kernel/init_task.o