[R510] AT command timeouts after warm boot

[keeramis]

Problem

Some of R510 warm boots take longer than expected and failing SLO criteria. In one of the runs with debugging enabled, it is noticed a run that took 115sec during warm boot (expected <30s) was blocked at ATI9 command for 90sec (which is default AT command timeout) and was causing the additional delays. It's next command though, works.

Solution

Add additional AT-OK checks at init() which helps if there was an internal race condition that could block the AT interface.

Steps to Test

Example App

Apply warm resets on any test app where cellular is initialized

References

SC90828

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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)