Allow the user to set which antenna to use on the sub device

So .. from the terminal, "-A RX2", would set the device to use the RX2
antenna.The UHD manual will define your options. If the user puts in a
bad selection, it will complain and list the options.

src/python/uhd_modes.py

@@ -63,6 +63,8 @@ def __init__(self, options, args, queue):
       if(options.rx_subdev_spec is None):
         options.rx_subdev_spec = ""
       self.u.set_subdev_spec(options.rx_subdev_spec)
+      if not options.antenna is None:
+        self.u.set_antenna(options.antenna)
 
       self.u.set_samp_rate(rate)
       rate = int(self.u.get_samp_rate()) #retrieve actual
@@ -114,6 +116,8 @@ def tune(self, freq):
   parser = OptionParser(option_class=eng_option, usage=usage)
   parser.add_option("-R", "--rx-subdev-spec", type="string",
           help="select USRP Rx side A or B", metavar="SUBDEV")
+  parser.add_option("-A", "--antenna", type="string",
+          help="select which antenna to use on daughterboard")
   parser.add_option("-f", "--freq", type="eng_float", default=1090e6,
                       help="set receive frequency in Hz [default=%default]", metavar="FREQ")
   parser.add_option("-g", "--gain", type="int", default=None,