Sierra Wireless modems need the string '$GPS_START' to be sent to the
GPS tty device as only then the modem firmware starts emitting
NMEA-0183 sentences.
Add an option 'sierragpsstart' to kplex' serial driver to support that
quirk as kplex can be very useful to spread GPS data over the network
while also supplying 'ugps' using a PTY, allowing for correct system
time to be set automatically on boot up from GPS.

This patch is also PR'ed at the upstream project:
https://github.com/stripydog/kplex/pull/54

Signed-off-by: Daniel Golle <daniel@makrotopia.org>