gpsd: add readonly option to uci

gpsd: add readonly option to uci

The 'readonly' option tells the GPSD that it is not allowed to write to the
GPS TTY, only reading the NMEA data stream is allowed. This option of the
GPSD must be set for my mobile radio device MV31, otherwise the GPSD can not
read the NMEA data stream on the GPS TTY because the GPSD blocks during
plugin probing.

gpsd:PROG: CORE: Probing "Garmin USB binary" driver...
gpsd:PROG: CORE: Probe not found "Garmin USB binary" driver...
gpsd:PROG: CORE: Probing "GeoStar" driver...
gpsd:PROG: Sent GeoStar packet id 0xc1

Signed-off-by: Florian Eckert <fe@dev.tdt.de>