USRP addressess and clock sources can be configured in the configuration file…

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