device-generic-x86_64: add oem-specific subpackages

Note: testing this currently requires these patches to pmbootstrap: https://lists.sr.ht/~postmarketos/pmbootstrap-devel/patches/48226

This uses oem-specific packages for configuring Intel and AMD systems, and installs them all by default for new images using pmbootstrap's pmb_recommends feature.

The intention is to provide pre-built images with support for Intel and AMD already enabled/configured, and users can remove an unwanted set of packages (e.g., apk del device-generic-x86_64-intel when on an AMD system) if they want to.

Upgrading existing systems should work, in that I haven't observed any apk conflicts with it. But you do need to manually install the new oem-specific package if you need it (e.g. oem-intel)

fixes #2484 (closed)

cc @jenneron, you helped with the idea for this design, and I'd like your feedback on how it's implemented here since we'll probably want to do something like this in other "generic" device packages?

cc @ollieparanoid on the new use of pmb_recommends (and the pmb patches, which I'll submit to the mailing list once I have some more feedback that this is on the right track 😅)