[boron] forward compatibility with 460800 baudrate on SARA R4

[avtolstoy]

Problem

We want to increase the baudrate on SARA R4-based devices to improve OTA times, however compared to all the other modems, AT+IPR setting on SARA R4 is persistent (!).

We need to have an intermediate release in 1.5.x line supporting both baudrates and changing the persistent value to 115200 in order to support downgrades from 2.x.

Solution

Try to talk to the modem on both 460800 and 115200 and change persistent value to 115200.

Steps to Test

1. Force LTE Boron/BSoM modem to use 460800 (AT+IPR=460800), power down device completely, flash system-part out of this branch
2. The device should connect to the cloud
3. Downgrade to 1.5.1, the device should connect to the cloud

Example App

N/A

References

• [CH54763]

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Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)

[keeramis]

Tested on

• OK Older BoronLTE cell version 02.00 --> picks up 115200
• OK Newer BoronLTE cell version 02.04 --> picks up 115200 when 115200 configured on cell radio
• OK Newer BoronLTE cell version 02.04 --> tests with 115200 and when it fails, moves to 460800 and gets to work