boards: slwstk6220a: remove support (so it can be replaced)

[basilfx]

Contribution description

This PR removes support for the stand-alone version of the SLWSTK6220a, so that it can be replaced by the EFM32 port.

After this gets merged, LOSTANDFOUND.md will be updated via separate PR, then another PR will be opened to add support using the EFM32 port. Then, there will be no two different EFM32 ports anymore.

Although it will be replaced, I think it's a good idea to list it in LOSTANDFOUND.md to highlight the stand-alone version.

Testing procedure

Confirm that it is removed and everything builds.

Issues/PRs references

None.

[PeterKietzmann]

@basilfx I have absolutely no idea about the status of EFM32 or what slwstk6220a is :-) but I assume this PR could be easily merged, right? Can you give a quick update on the current situation in RIOT?

[basilfx]

I dont know the state of the slwstk6220a. I don't own this board, so never tested it :-) But I think the new port is more superior, and at least contains all the features of this port.

I've rebased this PR.

[PeterKietzmann]

@basilfx now after some time has passed I think it might be a good idea to have the changes for re-integrating SLWSTK6220a directly available before removing it. What do you think? However, reintegration needs to be tested and I still don't have the hardware so we would need to find someone (maybe @haukepetersen as the initial author) to test it then.

[basilfx]

That's fine with me. I once talked to @haukepetersen and understood he had three SLWSTK6220a boards, and could send me one :-)

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[tcschmidt]

Is this ready for testing @haukepetersen ?

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This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. If you want me to ignore this issue, please mark it with the "State: don't stale" label. Thank you for your contributions.

[basilfx]

Rebased this PR. Should still be an easy one to merge.

[benpicco]

I take https://github.com/basilfx/EFM2RIOT/tree/develop will bring the replacement?

[benpicco]

looking forward for the version from EFM2RIOT along with it's siblings.

That one only needs to be adapted to the latest Kconfig changes.