[Ovmsdev] Integration of Simcom 7670 and changes to Simcom Powering

[Info Zeitnitz]

Dear all,
I finished to integrate the Simcom 7670 modem into the OVMS code.
See https://github.com/zbchristian/Open-Vehicle-Monitoring-System-3.git branch update_cellular.

Modem Powering
===============
I had problems to get the 7670 working with the current code. The modem was stuck in a power cycle loop.

I checked the Simcom documentation for the 4 models 5360, 7000 series, 7600, 7670 and the powering procedure via the PWRKEY pin is the same, but with different on/off timing. Depending on the 
hardware, the PWRKEY can be inverted as well. This is the case for my Lilygo TCall board, but I think not for the OVMS HW versions. At least version 3.1 connects the corresponding MAX7317 pin directly 
to the modem.

Assuming, that all hardware versions use the PWRKEY procedure for powering, I streamlined the code and moved the corresponding functions into separate files (simcom_powering.cpp and 
simcom_powering.h). This avoids the duplication of the code.

All this leads to substantial changes in ovms_cellular and ovms_cellular_modem_driver as well.

The code works now perfectly for the 7670, but testing is required for all other models.
If my assumptions about the OVMS HW versions are not correct, or the modems do not work as documented, some modifications might be needed.

GNSS Location
============
The 7670 no longer accepts the currently used AT commands to obtain the location  as a GRMC sentence. So, I added code to the new 7670 class to request the location via the AT+CGNSSINFO command.
Since this has to be send explicitly again and again,  I implemented a corresponding call in the StatusPoller of the 7670.
All the NMEA related code has been adapted to handle the corresponding CGNSSINFO response.


In conclusion, before doing a pull request, it would make sense that the code is tested with the other hardware and modem models.

So, tell me what you think and if the general strategy of the mods make sense to you.

Best regards
   Christian