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/mips/Makefile b/arch/mips/Makefile
index 64512ab..6b663be 100644
--- a/arch/mips/Makefile
+++ b/arch/mips/Makefile
@@ -608,7 +608,7 @@
   endif
 endif
 
-AFLAGS		+= $(cflags-y)
+KBUILD_AFLAGS	+= $(cflags-y)
 KBUILD_CFLAGS	+= $(cflags-y) \
 			-D"VMLINUX_LOAD_ADDRESS=$(load-y)"