Cellular: ALT1250: Store RAT to permament memory

[kivaisan]

Summary of changes

Save RAT selection to modem's permanent memory.

Impact of changes

This avoid unnecessary changing of RAT on every reboot if it is different than the default saved in modem's permanent memory.

Migration actions required

Documentation

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] 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)
[X] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

@ARMmbed/mbed-os-wan

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[ciarmcom]

@kivaisan, thank you for your changes.
@ARMmbed/mbed-os-wan @ARMmbed/mbed-os-maintainers please review.

[mbed-ci]

Test run: FAILED

Summary: 1 of 6 test jobs failed
Build number : 2
Build artifacts

Failed test jobs:

• jenkins-ci/mbed-os-ci_greentea-test

[0xc0170]

test restarted