SoapySDR support

There is no time stamp handling. It is just TX'ing in advance and
synchronous to received RX rate.

src/common/soapy.h

@@ -0,0 +1,7 @@
+
+int soapy_open(const char *device_args, double tx_frequency, double rx_frequency, double rate, double rx_gain, double tx_gain);
+void soapy_close(void);
+int soapy_send(float *buff, int num);
+int soapy_receive(float *buff, int max);
+int soapy_get_inbuffer(void);
+