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/powerpc/Makefile b/arch/powerpc/Makefile
index 5130984..462bb89 100644
--- a/arch/powerpc/Makefile
+++ b/arch/powerpc/Makefile
@@ -76,7 +76,7 @@
 CFLAGS-$(CONFIG_PPC64)	:= -mminimal-toc -mtraceback=none  -mcall-aixdesc
 CFLAGS-$(CONFIG_PPC32)	:= -Iarch/$(ARCH) -ffixed-r2 -mmultiple
 CPPFLAGS	+= $(CPPFLAGS-y)
-AFLAGS		+= $(AFLAGS-y)
+KBUILD_AFLAGS	+= $(AFLAGS-y)
 KBUILD_CFLAGS	+= -msoft-float -pipe $(CFLAGS-y)
 CPP		= $(CC) -E $(KBUILD_CFLAGS)
 
@@ -125,7 +125,7 @@
 cpu-as-$(CONFIG_E500)		+= -Wa,-me500
 cpu-as-$(CONFIG_E200)		+= -Wa,-me200
 
-AFLAGS += $(cpu-as-y)
+KBUILD_AFLAGS += $(cpu-as-y)
 KBUILD_CFLAGS += $(cpu-as-y)
 
 head-y				:= arch/powerpc/kernel/head_$(CONFIG_WORD_SIZE).o