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>

Documentation/kbuild/makefiles.txt

@@ -1099,7 +1099,7 @@ When kbuild executes, the following steps are followed (roughly):
 	specified options when building the target vmlinux.lds.
 
 	When building the *.lds target, kbuild uses the variables:
-	CPPFLAGS	: Set in top-level Makefile
+	KBUILD_CPPFLAGS	: Set in top-level Makefile
 	EXTRA_CPPFLAGS	: May be set in the kbuild makefile
 	CPPFLAGS_$(@F)  : Target specific flags.
 	                  Note that the full filename is used in this