kbuild: enable 'make CFLAGS=...' to add additional options to CC

The variable CFLAGS 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 CFLAGS with KBUILD_CFLAGS all over the
tree and enabling one to use:
make CFLAGS=...
to specify additional gcc commandline options.

One usecase is when trying to find gcc bugs but other
use cases has been requested too.

Patch was tested on following architectures:
alpha, arm, i386, x86_64, mips, sparc, sparc64, ia64, m68k

Test was simple to do a defconfig build, apply the patch and check
that nothing got rebuild.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

arch/um/Makefile-i386

diff --git a/arch/um/Makefile-i386 b/arch/um/Makefile-i386
index c9f1c5b..26bc786 100644
--- a/arch/um/Makefile-i386
+++ b/arch/um/Makefile-i386
@@ -15,7 +15,7 @@
 
 ifeq ("$(origin SUBARCH)", "command line")
 ifneq ("$(shell uname -m | sed -e s/i.86/i386/)", "$(SUBARCH)")
-CFLAGS			+= $(call cc-option,-m32)
+KBUILD_CFLAGS		+= $(call cc-option,-m32)
 AFLAGS			+= $(call cc-option,-m32)
 LINK-y			+= $(call cc-option,-m32)
 UML_OBJCOPYFLAGS	+= -F $(ELF_FORMAT)
@@ -36,4 +36,4 @@
 # an unresolved reference.
 cflags-y += -ffreestanding
 
-CFLAGS += $(cflags-y)
+KBUILD_CFLAGS += $(cflags-y)