Remove nRF51 targets

[MarceloSalazar]

Summary of changes

This is another of a series of PRs (others #12775, #12810, #12864, #12935) to remove targets that are not supported or retired as explained in recent blog post.

Impact of changes

The following public nRF51 targets are being removed:

• NRF51822
• NRF51_DK
• NRF51_MICROBIT

Migration actions required

• The targets listed above continue to be available in the 5.15 branch.

Documentation

• None.
  We'll update os.mbed.com/platforms with the list of supported platforms for Mbed 6

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Pull request type

[] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[X] 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

Expect regressions tests to be done by CI as part of this PR.

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Reviewers

@ARMmbed/mbed-os-maintainers

[ciarmcom]

@MarceloSalazar, thank you for your changes.
@ARMmbed/mbed-os-storage @ARMmbed/mbed-os-hal @ARMmbed/mbed-os-tools @ARMmbed/mbed-os-maintainers @ARMmbed/mbed-os-test @ARMmbed/mbed-os-core please review.

[ghost]

so which other nrf5x targets are being removed? I also think linking to that blog post isn't enough, since it lists a few different reasons which don't apply to every board you're removing.

[mbed-ci]

Test run: SUCCESS

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

[0xc0170]

I also think linking to that blog post isn't enough, since it lists a few different reasons which don't apply to every board you're removing.

Can you elaborate what we should document / change ? regarding the reason for being removed.

[ghost]

Removed for low usage isn't the same as being removed because the hardware is obsolete.

EDIT: afaict, the bbc:microbit isn't either of those, so it must even be a different reason than those.

[ghost]

I still want to know how far this removal is going, in case in need to switch away from mbed-os.

[MarceloSalazar]

Other nrf5x targets are listed here: #12810, #12864

All of them fall in at least one of the categories shown in the blog post. The most common is that they're not officially supported in Mbed OS 6+

Note this doesn't mean that can't be used. It's being done in a slightly different way. See examples in forums:
https://forums.mbed.com/c/custom-and-community-boards/

More info in this blog post
https://os.mbed.com/blog/entry/Custom-and-community-board-support/

[0xc0170]

I'll merge this one as agreed internally earlier