Cellular: Add CellularDevice::init_module API to be called at startup

[AriParkkila]

Description

Added a new API CellularDevice::init_module to initialize cellular module. This function need to be called right after cellular module is ready to make any module specific initialization.

Pull request type

[ ] Fix
[ ] Refactor
[ ] New target
[X] Feature
[ ] Breaking change

[AriParkkila]

@jarvte please review

[0xc0170]

Please split functional changes from style changes ( 2 separate commits)

[AriParkkila]

Split functional changes from style changes in separate commits.

@jarvte please re-review

[AriParkkila]

@0xc0170 please rereview

[AriParkkila]

@0xc0170 is there something still to do?

Changes addressed.

[cmonr]

@AriParkkila Nope! Thanks for the split.

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2793
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7667/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2423
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7667/

[mbed-ci]

Test : FAILURE

Build number : 2528
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7667/2528

[AriParkkila]

@cmonr ci-morph-test is failing with "mbedgt: no platform/target matching tests were found!". That's unlikely due to this change?

[adbridge]

Looks like this has failed due to some networking issues in CI. Suggest we re-run this once the CI is less heavily loaded.

[cmonr]

/morph test

[mbed-ci]

Test : SUCCESS

Build number : 2566
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7667/2566