Added support for RetroPlayer builds

To compile:
$ PROJECT=Generic ARCH=i386 XBMC=retroplayer make release

Conflicting patches (updated to work with retroplayer):
* xbmc-602-add_as.xml_busy_dialog_delay_control.patch
* xbmc-999.02-rpipower.patch
Fix XBMC compile error: use internal FFmpeg

PR:2878 (https://github.com/xbmc/xbmc/pull/2878) replaced deprecated CodecID with AVCodecID, but apparently OpenELEC's FFmpeg doesn't provide AVCodecID so disable OpenELEC's FFmpeg and use XBMC's internal FFmpeg.