kbuild: enable 'make CPPFLAGS=...' to add additional options to CPP

The variable CPPFLAGS is a wellknown variable and the usage by
kbuild may result in unexpected behaviour.

This patch replace use of CPPFLAGS with KBUILD_CPPFLAGS all over the
tree and enabling one to use:
make CPPFLAGS=...
to specify additional CPP commandline options.

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/ppc/Makefile b/arch/ppc/Makefile
index dd63ccb..95894ef 100644
--- a/arch/ppc/Makefile
+++ b/arch/ppc/Makefile
@@ -22,7 +22,7 @@
 
 LDFLAGS_vmlinux	:= -Ttext $(KERNELLOAD) -Bstatic
 # The -Iarch/$(ARCH)/include is temporary while we are merging
-CPPFLAGS	+= -Iarch/$(ARCH) -Iarch/$(ARCH)/include
+KBUILD_CPPFLAGS	+= -Iarch/$(ARCH) -Iarch/$(ARCH)/include
 KBUILD_AFLAGS	+= -Iarch/$(ARCH)
 KBUILD_CFLAGS	+= -Iarch/$(ARCH) -msoft-float -pipe \
 		-ffixed-r2 -mmultiple