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>