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/x86/boot/Makefile b/arch/x86/boot/Makefile
index 646292a..d6ed8e5 100644
--- a/arch/x86/boot/Makefile
+++ b/arch/x86/boot/Makefile
@@ -61,7 +61,7 @@
 			$(call cc-option, -fno-unit-at-a-time)) \
 		   $(call cc-option, -fno-stack-protector) \
 		   $(call cc-option, -mpreferred-stack-boundary=2)
-AFLAGS		:= $(KBUILD_CFLAGS) -D__ASSEMBLY__
+KBUILD_AFLAGS	:= $(KBUILD_CFLAGS) -D__ASSEMBLY__
 
 $(obj)/zImage:  IMAGE_OFFSET := 0x1000
 $(obj)/zImage:  EXTRA_AFLAGS := $(SVGA_MODE) $(RAMDISK)