gnuradio : Use PACKAGECONFIG so users can tweeak build options.

I hear from users that they are fiddling with the recipe to build gnuradio
with different options for various purposes. Start adding PACKAGECONFIG
support to reduce user editing of the recipe.

Currently these options are supported:

uhd : enable support for UHD based hardware.
grc: Build gnuradio companion.
qtgui: Build the qtgui component.

Signed-off-by: Philip Balister <philip@balister.org>