Enable disabling target files for custom targets

[multiplemonomials]

Summary of changes

This MR adds a feature for users to disable files that are part of one of the MCU targets (e.g. mbed-stm32l4 or mbed-stm32l452xe. It's as simple as:

mbed_disable_mcu_target_file(mbed-stm32l452xe system_clock.c)

Then, you can add your own system_clock.c as part of a custom target. No fuss, no muss!

Impact of changes

It's now possible to replicate the 'override' functionality that custom targets had in Mbed CLI 1 and mbed-cmake!

Migration actions required

None

Documentation

This is documented through the mbed-ce-custom-targets example project.

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Pull request type

[] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[X] 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)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[X] Tests / results supplied as part of this PR

I tested this locally by confirming that the L452RE_CUSTOM_CLOCK target builds in the example repo, and system_clock.c from Mbed is not compiled as part of it.

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Reviewers

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[JojoS62]

good idea, only the file compare shows 'no newline at end of file'

[multiplemonomials]

Hmm, I think it didn't have one before either. Is having an ending newline a requirement? I don't think it has to be?

[multiplemonomials]

@JojoS62 @JohnK1987 did you have a chance to review this?