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/sh/Makefile b/arch/sh/Makefile
index cd8a411..0353296 100644
--- a/arch/sh/Makefile
+++ b/arch/sh/Makefile
@@ -92,7 +92,7 @@
 endif
 
 KBUILD_CFLAGS		+= -pipe $(cflags-y)
-AFLAGS		+= $(cflags-y)
+KBUILD_AFLAGS		+= $(cflags-y)
 
 head-y := arch/sh/kernel/head.o arch/sh/kernel/init_task.o