uhd: Add command line option for GPS reference

Unlike earlier versions of UHD, the current release (3.9.2)
does not automatically select on-board GPSDO as the reference
source. Modify the command line settings to allow explicit
selection of GPS in addition to the external setting.

Simultaneous GPS and external reference settingis disallowed.

Signed-off-by: Tom Tsou <tom.tsou@ettus.com>
5 files changed