x300_dboard_iface: added UHD_UNUSED() macro to unused parameters in set_clock_enabled()

* Removes GCC warning about unused parameters
Merge branch 'maint'
Added missing pure virtual destructors to base classes
Updated ABI version string
Give user the option to ignore daughterboard's calibration file at runtime

* Add "ignore-cal-file" to the uhd::device_addr_t arguments
* Added documentation for new feature