Fix issue when TDBStore has varying erase sizes between areas.

[jarlamsa]

Summary of changes

In some cases, it is possible that every erase unit in area 0
has the same size, but they are still different than in area 1.
Remove the flag for varying erase sizes and instead check every time
from flash, what is the erase size of the current unit.

Documentation

None

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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-storage
@soleilplanet

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[ciarmcom]

@jarlamsa, thank you for your changes.
@soleilplanet @ARMmbed/mbed-os-storage @ARMmbed/mbed-os-maintainers please review.

[VeijoPesonen]

Looks good to me

[0xc0170]

CI started

[mbed-ci]

Test run: FAILED

Summary: 2 of 7 test jobs failed
Build number : 1
Build artifacts

Failed test jobs:

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

[jarlamsa]

@0xc0170 The errors seem to be CI-related, please restart.

[0xc0170]

CI restarted

[adbridge]

@jarlamsa in order to bring this cleanly to the mbed-os-5.15 branch could we please have an equivalent PR directly to that branch. We are getting PRs directly to that branch with functionality that is either not supported on Master or has deviated and thus we have a diverging code base. By creating an equivalent PR to this directly against that branch will enable us to ensure that the PR lands in time and cleanly. Thanks. We will then move the release version label over to that PR.

[jarlamsa]

@adbridge sure, I'll provide that soon.