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/m68knommu/Makefile b/arch/m68knommu/Makefile
index 1c1fbb6..92227aa 100644
--- a/arch/m68knommu/Makefile
+++ b/arch/m68knommu/Makefile
@@ -102,7 +102,7 @@
 cflags-$(CONFIG_M68VZ328)	:= -m68000
 cflags-$(CONFIG_M68360)		:= -m68332
 
-AFLAGS += $(cflags-y)
+KBUILD_AFLAGS += $(cflags-y)
 
 KBUILD_CFLAGS += $(cflags-y)
 KBUILD_CFLAGS += -D__linux__