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/avr32/Makefile b/arch/avr32/Makefile
index dc6bc01..9883c8b 100644
--- a/arch/avr32/Makefile
+++ b/arch/avr32/Makefile
@@ -11,14 +11,14 @@
 
 KBUILD_DEFCONFIG	:= atstk1002_defconfig
 
-CFLAGS		+= -pipe -fno-builtin -mno-pic
+KBUILD_CFLAGS	+= -pipe -fno-builtin -mno-pic
 AFLAGS		+= -mrelax -mno-pic
 CFLAGS_MODULE	+= -mno-relax
 LDFLAGS_vmlinux	+= --relax
 
 cpuflags-$(CONFIG_CPU_AT32AP7000)	+= -mcpu=ap7000
 
-CFLAGS		+= $(cpuflags-y)
+KBUILD_CFLAGS	+= $(cpuflags-y)
 AFLAGS		+= $(cpuflags-y)
 
 CHECKFLAGS	+= -D__avr32__ -D__BIG_ENDIAN