Cellular: Fix BG96 power on and connect

[AriParkkila]

Description

Fixed BG96 power up/down, disconnect at shutdown(), and retry at cellular state-machine calling init() to make dis/connect more robust. Also some log prints changed to be more informative and less noisy.

Pull request type

[x] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

Reviewers

@mirelachirica @jarvte

Release Notes

[ciarmcom]

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

[0xc0170]

@AriParkkila The latest merge caused a conflict, please rebase

[AriParkkila]

@0xc0170 It's rebased now.

[0xc0170]

CI started

[mbed-ci]

Test run: FAILED

Summary: 1 of 4 test jobs failed
Build number : 1
Build artifacts

Failed test jobs:

• jenkins-ci/mbed-os-ci_unittests

[AriParkkila]

unittests are fixed now.

[0xc0170]

CI started

[mbed-ci]

Test run: FAILED

Summary: 2 of 4 test jobs failed
Build number : 2
Build artifacts

Failed test jobs:

• jenkins-ci/mbed-os-ci_build-IAR
• jenkins-ci/mbed-os-ci_build-ARM

[0xc0170]

CI restarted (internal fault)

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 3
Build artifacts

[0xc0170]

@AriParkkila Can you rebase pls? There's conflict caused by earlier patch.

[AriParkkila]

@0xc0170 This is rebased now.

[0xc0170]

CI restarted

[mbed-ci]

Test run: FAILED

Summary: 1 of 4 test jobs failed
Build number : 4
Build artifacts

Failed test jobs:

• jenkins-ci/mbed-os-ci_build-ARM

[0xc0170]

CI restarted

[mbed-ci]

Test run: FAILED

Summary: 1 of 11 test jobs failed
Build number : 5
Build artifacts

Failed test jobs:

• jenkins-ci/mbed-os-ci_cloud-client-test

[0xc0170]

cloud client restarted (internal fault)