clean carrier emission between bitstream transfer and first PPS + PLL drift

[jmfriedt]

The X310 emits a clean carrier between the time the new bitstream is transferred. While this could be considered as a feature, it induces unwanted transmission to the upconverter.
1/ remove the unwanted clean carrier emission until a first PPS is received
2/ possibly add a GPIO input to continuously generate the clean carrier (useful?)

[jmfriedt]

Also observed on the CMOD-A7 FPGA output (carrier emission between Enable and 1-PPS rising triggering the PRN modulation)