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/ppc/Makefile b/arch/ppc/Makefile
index 43802e9..dd63ccb 100644
--- a/arch/ppc/Makefile
+++ b/arch/ppc/Makefile
@@ -23,7 +23,7 @@
 LDFLAGS_vmlinux	:= -Ttext $(KERNELLOAD) -Bstatic
 # The -Iarch/$(ARCH)/include is temporary while we are merging
 CPPFLAGS	+= -Iarch/$(ARCH) -Iarch/$(ARCH)/include
-AFLAGS		+= -Iarch/$(ARCH)
+KBUILD_AFLAGS	+= -Iarch/$(ARCH)
 KBUILD_CFLAGS	+= -Iarch/$(ARCH) -msoft-float -pipe \
 		-ffixed-r2 -mmultiple
 
@@ -45,7 +45,7 @@
 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)
 
 # Default to the common case.