Added support for RetroPlayer builds

Before compiling, you need to pull the RetroPlayer branch into a tarball:
$ cd tools/mkpkg
$ ./mkpkg_xbmc-retroplayer

To compile for project Generic (32-bit):
$ time PROJECT=Generic ARCH=i386 XBMC=retroplayer make release

Conflicting patches (updated for RetroPlayer):
* xbmc-995.01-fernetmenta-fixes-b3b79dd.patch
* xbmc-995.02-dont-set-_NET_WM_STATE_FULLSCREEN.patch (unmodified)
* xbmc-995.03-keymap.patch (unmodified)
* xbmc-999.00-service-addons-use-a-wrapper-to-setup-systemd.patch