GPS: support for 230400 baud
UBX CFG: add code to clear GPS configuration

This code is disabled by default but available if needed.
UBX CFG: use NAV-SOL instead of NAV-STATUS

We use the NAV-SOL message because it provides the
number of satellites as well as the fix status. The
NAV-STATUS is missing the number of satellites.

However, this is a 50 byte as opposed to 12 byte
message so that is non trivial.
UBX CFG: DOP and SOL must happen at full rate

Otherwise the GPS module thinks (correctly) the data
is stale and switches to "NoGPS".
UBX CFG: fix CFG-RATE message