kbuild: allow assignment to {A,C}FLAGS_KERNEL on the command line

It is now possible to assign options to AS and CC
on the command line - which is only used for built-in code.

{A,C}FLAGS_KERNEL was used both in the top-level Makefile
in the arch makefiles, thus users had no way to specify
additional options to AS, CC without overriding
the original value.

Introduce a new set of variables KBUILD_{A,C}FLAGS_KERNEL
that is used by arch specific files and free up
{A,C}FLAGS_KERNEL so they can be assigned on
the command line.

All arch Makefiles that used the old variables has been updated.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Hirokazu Takata <takata@linux-m32r.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>
diff --git a/arch/m32r/Makefile b/arch/m32r/Makefile
index 14a3c23..8ff5ba0 100644
--- a/arch/m32r/Makefile
+++ b/arch/m32r/Makefile
@@ -12,7 +12,7 @@
 LDFLAGS_vmlinux	:=
 
 KBUILD_CFLAGS += -pipe -fno-schedule-insns
-CFLAGS_KERNEL += -mmodel=medium
+KBUILD_CFLAGS_KERNEL += -mmodel=medium
 KBUILD_CFLAGS_MODULE += -mmodel=large
 
 ifdef CONFIG_CHIP_VDEC2