clarify comments

Signed-off-by: Hiroshi Miura <miurahr@linux.com>
use NmeaState object to handle a notification timing

Signed-off-by: Hiroshi Miura <miurahr@linux.com>
nmea: update comment to clarify logic

Signed-off-by: Hiroshi Miura <miurahr@linux.com>
check should be used RMC and GLL sentence

Signed-off-by: Hiroshi Miura <miurahr@linux.com>
Merge branch 'cleanup_gps_and_nmea_status'