[BUILD] Miscellaneous build-system fixes and tweaks.

[BUILD] Miscellaneous build-system fixes and tweaks.

* Don't delete non-existent %.tar.bz2 in buildconfigs/Rules.mk's %-mrproper
* Add a mrpropper target to buildconfigs/mk.linux-2.6-xen, and
  have this delete the local tarball
  - This should also delete the local patch, if there is one, but I am not
    sure how to do this with the current incarntation of the code, and I
    will post a subsequent patch which makes this easier and effects this
    chang. As there currently is no local patch this is not a big deal at
    this moment. (Horms)
* Stop kclean from erroring out if the kernel directory doesn't exist
* Remove patches/*/.makedep in top level Makefile rather than in
  buildconfigs/Rules.mk, as calling rules in the latter causes
  patches/*/.make to be created (Magnus)

Signed-Off-By: Magnus Damm <magnus@valinux.co.jp>
Signed-Off-By: Horms <horms@verge.net.au>