Transceiver52M: Move reference select from compile time to database

Enabling the external reference on UHD devices through the configure
time switch is awkward. Use a database variable "TRX.Reference" with
'0' or '1' value for internal and external references respectively.
Use internal reference is no entry is defined.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
diff --git a/Transceiver52M/USRPDevice.h b/Transceiver52M/USRPDevice.h
index f74db47..dc86f0e 100644
--- a/Transceiver52M/USRPDevice.h
+++ b/Transceiver52M/USRPDevice.h
@@ -105,7 +105,7 @@
   USRPDevice(int sps, bool skipRx);
 
   /** Instantiate the USRP */
-  int open(const std::string &);
+  int open(const std::string &, bool);
 
   /** Start the USRP */
   bool start();