gstreamer1.0-omx: Target Raspberry Pi instead of Bellagio.

This changes the build slightly as there are some #ifdefs in there for Raspberry Pi.

Also the codec configuration file used by gstreamer1.0-omx codecs, /etc/xdg/gstomx.conf,  is set correctly to core-name=/usr/lib/libopenmaxil.so

Change-Id: I2352ecabfd053717d9ccd2d22422e7d4b7588ce4
Signed-off-by: Alex J Lennon <ajlennon@dynamicdevices.co.uk>