bladeRF: Add XB-200 support

This commit adds support for the bladeRF XB-200 transverter expansion board. To
enable the expansion board and to allow the osmocom source to down to 0Hz,
parameter XB-200 has to be set. Additionally the XB-200 comes with 4 filter
banks that can be set via configuration option, xb200filt. The following values
are valid:
   "50M"     :  50MHz band
   "144M"    : 144MHz band
   "222M"    : 222MHz band
   [default] : Custom filter
bladeRF: auto filterbank selection

Setting xb200filt to "auto" causes the bladeRF sink and source blocks to
automatically select the appropriate filterbank based on center
frequency.