fixed the NMEA checksum switch

better string safety in device drivers for serial number and device name.
limit Serial number messages to max. 5 instead of 10. for each device
no toggle if 2. devices involved
Merge branch 'NMEA_Checksum_fix' of git://github.com/AlphaLima/LK8000 into ulli
Regression on unneeded checksum in device.cpp, already managed in individual devices