GPS: fixed the UBLOX parser to handle unknown messages

the parser was broken in several ways:

 1) when it received an unknown message it didn't update the ck_a and ck_b fields, so it
    thought the message had a bad checksum, which meant it got out of protocol sync

 2) the read() method would return false if the last message from the GPS was of an unknown type. 
    So we relied on the last msg always being one that we understand and want

 3) the parser considered any valid UBLOX message to be 'new data', whereas we only actually get 
    a new fix when we get box a new position and velned message

 4) the total message size per update is more than 128 bytes, but the serial port was opened 
    with only a 128 byte buffer, so we got corruption regularly