memtest86plus: Add at v7.00

[ermo]

Summary

Could be useful to have in the default boot menu and on install media.

Test Plan

(None yet -- this PR is for soliciting feedback)

Checklist

• [x] Package was built and tested against unstable

[ermo]

@silkeh What are the odds we could add a memtest boot entry by default for systems with this package installed and for the ISOs?

The organisation on-disk of this is just a draft; I added what I thought might be pertinent for now, but this is by no means intended to be the finished article...