kbuild/xfs: introduce fs/xfs/Kbuild

In kbuild the file named 'Kbuild' has precedence over the file named
Makefile. Utilise a file named Kbuild to include the 2.6 Makefile for xfs
- since the xfs people likes to keep their arch specific Makefiles separate.

With this patch xfs does no longer rely on the KERNELRELEASE components to be global.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>