kbuild: expand -I in KBUILD_CPPFLAGS

kbuild failed to expand include flags in KBUILD_CPPFLAGS
resulting in code like this in arch Makefiles:

ifeq ($(KBUILD_SRC),)
KBUILD_CPPFLAGS += -Iinclude/foo
else
KBUILD_CPPFLAGS += -I$(srctree)/include/foo
endif

Move use of LINUXINCLUDE into Makefile.lib to allow
us to expand -I directives of KBUILD_CPPFLAGS so
we can avoid the above code.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/Makefile b/Makefile
index 9a49960..3075586 100644
--- a/Makefile
+++ b/Makefile
@@ -336,7 +336,7 @@
                    -I$(srctree)/arch/$(hdr-arch)/include               \
                    -include include/linux/autoconf.h
 
-KBUILD_CPPFLAGS := -D__KERNEL__ $(LINUXINCLUDE)
+KBUILD_CPPFLAGS := -D__KERNEL__
 
 KBUILD_CFLAGS   := -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs \
 		   -fno-strict-aliasing -fno-common \
@@ -1638,7 +1638,7 @@
                   $(if $(KBUILD_MODULES),; rm -f $(MODVERDIR)/*)
 
 a_flags = -Wp,-MD,$(depfile) $(KBUILD_AFLAGS) $(AFLAGS_KERNEL) \
-	  $(NOSTDINC_FLAGS) $(KBUILD_CPPFLAGS) \
+	  $(NOSTDINC_FLAGS) $(LINUXINCLUDE) $(KBUILD_CPPFLAGS) \
 	  $(modkern_aflags) $(EXTRA_AFLAGS) $(AFLAGS_$(basetarget).o)
 
 quiet_cmd_as_o_S = AS      $@