gr-osmosdr: new package

Add support for gr-osmosdr - a GNU Radio block for interfacing with various
radio hardware.

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
[Thomas:
 - instead of selecting BR2_PACKAGE_GNURADIO, use "depends
   on". Indeed, it is fairly obvious for a user that some GNURadio
   extensions require GNURadio to be built. And it avoids the need to
   replicate all the complex dependencies of GNURadio.
 - remove comment that the Python support needs Python. It's fairly
   obvious, no?
 - remove comment about the thread dependency of
   BR2_PACKAGE_GR_OSMOSDR_RTLSDR, since anyway GNURadio already need
   threads. Yes, we sometimes still add such dependencies, but in the
   case of GNURadio, it's highly unlikely that GNURadio will ever
   loose the thread dependency.
 - use "NO" instead of "no" as the value for
   GR_OSMOSDR_SUPPORTS_IN_SOURCE_BUILD since we always use upper-case
   letters for the value of such booleans.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>