boards: add openlabs-kw41z-mini-256kib

[benpicco]

Contribution description

Add the 256k version of the openlabs-kw41z-mini.
They are identical to the openlabs-kw41z-mini except that it uses the mkw41z256vht4 instead of the mkw41z512vht4.

@benemorius says these were only sent out by mistake, but it's rather trivial to support them too and I dislike the idea of not using otherwise perfectly good hardware 😉

I also wanted to try how easy it is to support multiple boards that only differ in CPU.

We took a more elaborate approach with the different blackpill/bluepill versions which also only differ in flash size, but it turns out just providing the basic Makefiles is enough.

Testing procedure

Flash one of the 256k boards with this.

Issues/PRs references

needs #13522

[benemorius]

Oh, this is brilliant! I didn't know it could be done so efficiently. Thanks for that!

I forget whether I had resolved the deal with riotboot yet but I'll investigate.

[leandrolanzieri]

I'm checking the openlabs board that I have here to see if it is a 256 or a 512. With JLink I get from the SIM_SDID register the value 0x41571004. From that I can see that it has only 64Kb (following the reference manual p.270). According to the datasheet the only MKW41Z device with that amount of RAM is the MKW41Z256VHT4.

@benpicco @benemorius is this correct? is there a simple way to check which version I have?

[benpicco]

@leandrolanzieri you can check the print on the chip

#10846 (comment)

[leandrolanzieri]

@leandrolanzieri you can check the print on the chip

#10846 (comment)

Ok so it says M41W 8VT4 1N16S CTDFMD

[leandrolanzieri]

Changes look good, and work as expected. ACK.

@benpicco please squash.

[miri64]

Only peripherally related comment: the doc of these boards is broken in the rendered versions.

[benpicco]

I'll fix that in a separate PR.