STM32F0: create STM32F091xC target

[jeromecoutant]

Summary of changes

In order to ease custom targets with STMF091xC MCU,
a intermediate directory is created in TARGET_STM32F0

Fix #12009

@pilotak

Impact of changes

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

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[0xc0170]

@pilotak Please review, I start CI meanwhile

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
Build artifacts

[pilotak]

Perfect, thanks a lot.

@jeromecoutant well done for how fast you found it fixed it.

[pilotak]

by looking at the STM target directory all F0, F1 and F2 would have this problem too. Maybe this could be added on todo list...