STM32 custom target: create MCU_STM32xx for each family

[jeromecoutant]

Summary of changes

In order to help custom targets support, I propose to create in targets.json file,
for each STM32 family, a MCU_STM32xx intermediate configuration.

Now, when you create a custom target, you don't need to copy all common information that customer doesn't know,
only need to add "inherits": "MCU_STM32L4" for ex

Impact of changes

Should be none...

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

I started ST CI for this week end.

[] 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

@MarceloSalazar
@ARMmbed/team-st-mcd

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[ciarmcom]

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

[0xc0170]

@jeromecoutant Please review Travis failures

Also, can you add details to the commit message? Why are we doing this and how the new inheritance should look like.

[ladislas]

This is good news! As we are working on a custom board at the moment, everything that can make it easier is welcome! :)

[jeromecoutant]

@jeromecoutant Please review Travis failures

Please merge #13237 first

Also, can you add details to the commit message? Why are we doing this and how the new inheritance should look like.

Done

[jeromecoutant]

Let's restart travis ?
Thx

[0xc0170]

@jeromecoutant Can you rebase on top of master please?

[jeromecoutant]

@jeromecoutant Can you rebase on top of master please?

Done

[0xc0170]

Having families +1

[jeromecoutant]

Let's start CI ?

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 6 of 6 test jobs passed
Build number : 1
Build artifacts