Add support to flashing UBLOX AssistNow Offline data to GPS flash

[mmosca]

Easiest way is probably by using gpspassthrough.

After entering passthrough, configurator can use the UBX-MGA-FLASH-DATA command and wait for UBX-MGA-FLASH-ACK before sending next 512bytes.
Send UBX-MGA-FLASH-STOP , wait for UBX-MGA-FLASH-ACK.

Process described under "Flash-based AssistNow Offline" at https://developer.thingstream.io/guides/location-services/assistnow-user-guide#h.uvagy6xln2kf