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/scripts/Makefile.lib b/scripts/Makefile.lib
index 2837ada..95e6e0f 100644
--- a/scripts/Makefile.lib
+++ b/scripts/Makefile.lib
@@ -86,7 +86,7 @@
 
 _c_flags       = $(KBUILD_CFLAGS) $(EXTRA_CFLAGS) $(CFLAGS_$(basetarget).o)
 _a_flags       = $(KBUILD_AFLAGS) $(EXTRA_AFLAGS) $(AFLAGS_$(basetarget).o)
-_cpp_flags     = $(CPPFLAGS) $(EXTRA_CPPFLAGS) $(CPPFLAGS_$(@F))
+_cpp_flags     = $(KBUILD_CPPFLAGS) $(EXTRA_CPPFLAGS) $(CPPFLAGS_$(@F))
 
 # If building the kernel in a separate objtree expand all occurrences
 # of -Idir to -I$(srctree)/dir except for absolute paths (starting with '/').
@@ -106,11 +106,11 @@
 __cpp_flags     =                          $(call flags,_cpp_flags)
 endif
 
-c_flags        = -Wp,-MD,$(depfile) $(NOSTDINC_FLAGS) $(CPPFLAGS) \
+c_flags        = -Wp,-MD,$(depfile) $(NOSTDINC_FLAGS) $(KBUILD_CPPFLAGS) \
 		 $(__c_flags) $(modkern_cflags) \
 		 -D"KBUILD_STR(s)=\#s" $(basename_flags) $(modname_flags)
 
-a_flags        = -Wp,-MD,$(depfile) $(NOSTDINC_FLAGS) $(CPPFLAGS) \
+a_flags        = -Wp,-MD,$(depfile) $(NOSTDINC_FLAGS) $(KBUILD_CPPFLAGS) \
 		 $(__a_flags) $(modkern_aflags)
 
 cpp_flags      = -Wp,-MD,$(depfile) $(NOSTDINC_FLAGS) $(__cpp_flags)