package/gnuradio: add gr-uhd option

GNURadio has a block to use USRP, through UHD, to receive or transmit RF
signals.

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>