FX2 code has complications on Windows machine due to
shell misinterpreting the "/sbin/rmmod usbtest" string.
* path should not be hardcoded
* std::system error message means "possible success" (which is confusing, and contains little information)
* replaced std::system by matching syscall
* used #ifdef UHD_PLATFORM_LINUX to make checking & removal Linux-only
* OctoClock can communicate with UHD over Ethernet
* Can read NMEA strings from GPSDO and send to host
* Added multi_usrp_clock class for clock devices
* uhd::device can now filter to return only USRP devices or clock devices
* New OctoClock bootloader can accept firmware download over Ethernet
* Added octoclock_burn_eeprom,octoclock_firmware_burner utilities
* Added test_clock_synch example to show clock API
