Make SFN dependent on GPS time
somehow this commit didn't make it into develop although it was part of another MR it syncs the USRP time to GPS time when available and then derives the SFN from the USRP time.
somehow this commit didn't make it into develop although it was part of another MR it syncs the USRP time to GPS time when available and then derives the SFN from the USRP time.