trx-usrp1: make single daughterboard VTY configurable
Adds the usrp1-singledb enable/disable option to specify the daughterboard configuration on USRP1 devices. This will allow users to use single-daughterboard devices without compliling from source to use the --with-singledb configure option. Also removed some daughterboard configuration related code that wasn't being used. More info in the modified trx-backends.adoc
