The default setup with 100ms between GPS commands doesn't seem to be
always enough for the Engine activation step:
[1495016625.392972] (ttyACM1): --> 'AT^SGPSC="NMEA/Output","on"<CR>'
[1495016625.503885] (ttyACM1): <-- '<CR><LF>^SGPSC: "Nmea/Output","on"<CR><LF><CR><LF><CR><LF>OK<CR><LF>'
[1495016625.607650] (ttyACM1): --> 'AT^SGPSC="Power/Antenna","on"<CR>'
[1495016625.697862] (ttyACM1): <-- '<CR><LF>^SGPSC: "Power/Antenna","on"<CR><LF><CR><LF>OK<CR><LF>'
[1495016625.809393] (ttyACM1): --> 'AT^SGPSC="Engine","1"<CR>'
[1495016625.895970] (ttyACM1): <-- '<CR><LF>+CME ERROR: 767<CR><LF>'
We now setup up to 3 retries for the Engine activation step before
returning an error, and we also update to 2000ms the wait time before
the Engine activation command is run.
The AT^SGPSS command provides an easy way to just start/stop GPS, but
unfortunately it isn't supported by all Cinterion modems.
The AT^SGPSC command instead is more widely available but it requires
several steps to start and stop the different elements of the GPS
receiver.
Implement support for both, preferring AT^SGPSSS if available and
falling back to AT^SGPSC otherwise.
When checking for location capabilities, we will make sure AT^SGPSS is
supported and if it isn't we won't report GPS capabilities.
The location enable and disable paths are refactored to make it easier
to add possible new GPS commands to use instead of AT^SGPSS, if this
isn't supported (e.g. in the PLS8 devices).
The PHS8 in QMI-mode doesn't support GPS location retrieval via QMI, so we will
fallback to use the AT-based setup and the TTY for reading NMEA traces.
