USRP addressess and clock sources can be configured in the configuration file with the RU section with comma separated IP addresses in 'sdr_addrs' and with strings "internal"/"external"/"gpsdo" in 'clock_src'.

TODO: Currently the driver picks the two first RX and TX channels when nb_tx and nb_rx are both set to 2. If a developers wants to force usage of two devices, 'set_rx_subdev_spec' and 'set_tx_subdev_spec' can be used to indicate to the driver so that antennas from separate devices will be used. To rephrase: until 4 antennas are supported by OAI, testing with two devices, so that antennas from both devices are used, requires manual configuration of which subdevices' antennas are used.

Conflicts:
	targets/ARCH/USRP/USERSPACE/LIB/usrp_lib.cpp

targets/ARCH/COMMON/common_lib.h

@@ -199,6 +199,8 @@ typedef struct {
   double tx_bw;
   //! clock source 
   clock_source_t clock_source;
+  //! Manual SDR IP address
+  char *sdr_addrs;
   //! Auto calibration flag
   int autocal[4];
   //! rf devices work with x bits iqs when oai have its own iq format