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/arm/Makefile b/arch/arm/Makefile
index b762d23..0d450e7 100644
--- a/arch/arm/Makefile
+++ b/arch/arm/Makefile
@@ -32,11 +32,11 @@
 endif
 
 ifeq ($(CONFIG_CPU_BIG_ENDIAN),y)
-CPPFLAGS	+= -mbig-endian
+KBUILD_CPPFLAGS	+= -mbig-endian
 AS		+= -EB
 LD		+= -EB
 else
-CPPFLAGS	+= -mlittle-endian
+KBUILD_CPPFLAGS	+= -mlittle-endian
 AS		+= -EL
 LD		+= -EL
 endif