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/arm/Makefile b/arch/arm/Makefile
index 0b82865..b762d23 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -86,7 +86,7 @@
 
 # Need -Uarm for gcc < 3.x
 KBUILD_CFLAGS	+=$(CFLAGS_ABI) $(arch-y) $(tune-y) $(call cc-option,-mshort-load-bytes,$(call cc-option,-malignment-traps,)) -msoft-float -Uarm
-AFLAGS		+=$(CFLAGS_ABI) $(arch-y) $(tune-y) -msoft-float
+KBUILD_AFLAGS	+=$(CFLAGS_ABI) $(arch-y) $(tune-y) -msoft-float
 
 CHECKFLAGS	+= -D__arm__