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/h8300/Makefile b/arch/h8300/Makefile
index 330e05f..a556447 100644
--- a/arch/h8300/Makefile
+++ b/arch/h8300/Makefile
@@ -35,7 +35,7 @@
 KBUILD_CFLAGS += -g
 KBUILD_CFLAGS += -D__linux__
 KBUILD_CFLAGS += -DUTS_SYSNAME=\"uClinux\"
-AFLAGS += -DPLATFORM=$(PLATFORM) -DMODEL=$(MODEL) $(cflags-y)
+KBUILD_AFLAGS += -DPLATFORM=$(PLATFORM) -DMODEL=$(MODEL) $(cflags-y)
 LDFLAGS += $(ldflags-y)
 
 CROSS_COMPILE = h8300-elf-