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>
diff --git a/arch/arm/boot/compressed/Makefile b/arch/arm/boot/compressed/Makefile
index 6b8cbd6..5fde99f 100644
--- a/arch/arm/boot/compressed/Makefile
+++ b/arch/arm/boot/compressed/Makefile
@@ -87,7 +87,7 @@
 LDFLAGS_vmlinux += --defsym params_phys=$(PARAMS_PHYS)
 endif
 LDFLAGS_vmlinux += -p --no-undefined -X \
-	$(shell $(CC) $(CFLAGS) --print-libgcc-file-name) -T
+	$(shell $(CC) $(KBUILD_CFLAGS) --print-libgcc-file-name) -T
 
 # Don't allow any static data in misc.o, which
 # would otherwise mess up our GOT table