Add support for Telit Cinterion TX62

[multiplemonomials]

Summary of changes

This PR adds support to Mbed OS for the Telit (formerly Gemalto) Cinterion TX62 modem. This modem is used on the Arduino Portenta Cat.M1/NB IoT shield, and it's needed in order for the Arduino core to compile against Mbed. I adapted it from a series of patches that Arduino created, starting with this one. I have also tried to clean it up somewhat, so that the changes to classes outside of the Cinterion driver are minimized.

One thing I did change verseus the Arduino patches is I removed the ability to select the specific radio band that the modem operates on. This seems like it was kinda complicated to implement, and reading the modem datasheet, it says that it's perfectly capable of selecting its own bands based on what the cell towers in your area offer. So, not adding that unless there's an actual demonstrable reason why someone needs it.

Another note is that the "CMUX" support works together with a bunch of custom logic that Arduino implements in their serial code to basically mux one serial port into three. That logic currently doesn't exist in Mbed, and it would probably take a fair bit of work to port it over in a non janky way. So, for now I am merging their patch that adds a way to turn on CMUX, even though there's no way to actually use it with plain Mbed.

Impact of changes

• Support added for Telit Cinterion TX62 modem

Migration actions required

Documentation

None

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Pull request type

[] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[X] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

Unfortunately I can't test this at present as I don't have TX62 hardware available...

[zhiyong-ft]

Did you test it with an actual module? I don't have one to test but the code looks clean (enough to me). Also asking from experience with EC25, does this module/code support query for signal strength? The code in Mbed-OS for EC25 only supports query for RSSI before connect().

[multiplemonomials]

Unfortunately not @zhiyong-ft . I don't have actual hardware and I have zero experience with cellular modules before this PR... that's why I asked for you to review it as I know you have used stuff like this before 🙏

[zhiyong-ft]

Let me see if I can get some hardware to test. It is not a good idea to merge untested code.