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.