gpsd: override gpsd.service to use GPS_DEVICES from /etc/default/gpsd

Without that fix gpsd starts without using the machine specific
  gps serial port.

On some devices like the GTA04, not only that makes GPS not work,
  but that also prevent suspend because GPS has to be off in order
  for suspend to succeed(and the program powering off the GPS
  has to know if the GPS is on or off by using gpsd).

Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@no-log.org>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>