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/scripts/Makefile.modinst b/scripts/Makefile.modinst
index 85d6494..23fd1bd 100644
--- a/scripts/Makefile.modinst
+++ b/scripts/Makefile.modinst
@@ -5,7 +5,7 @@
 .PHONY: __modinst
 __modinst:
 
-include scripts/Makefile.lib
+include scripts/Kbuild.include
 
 #