kbuild: introduce Kbuild.include

Kbuild.include is a placeholder for definitions originally present in
both the top-level Makefile and scripts/Makefile.build.
There were a slight difference in the filechk definition, so the most videly
used version was kept and usr/Makefile was adopted for this syntax.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
---
diff --git a/usr/Makefile b/usr/Makefile
index 248d555..e2129cb 100644
--- a/usr/Makefile
+++ b/usr/Makefile
@@ -27,7 +27,7 @@
 filechk_initramfs_list = $(CONFIG_SHELL) \
  $(srctree)/scripts/gen_initramfs_list.sh $(gen_initramfs_args) $(quotefixed_initramfs_source)
 
-$(obj)/initramfs_list: FORCE
+$(obj)/initramfs_list: $(obj)/Makefile FORCE
 	$(call filechk,initramfs_list)
 
 quiet_cmd_cpio = CPIO    $@