main/postmarketos-mkinitfs: add modprobe subpkgs

This adds two new subpackages, postmarketos-mkinitfs-modprobe-lts and postmarketos-mkinitfs-modprobe-edge, that include /sbin/modprobe in the initfs when one of the Alpine kernels (linux-lts or linux-edge) are also installed.

I couldn't find any documentation for the install_if property but in browsing the apk source it seemed as if the value was treated as a plain array of conditions without support for logical operators. Therefore, I simply created two separate subpackages to install /sbin/modprobe for both the LTS and the edge kernel.

I didn't test this on a physical device but in qemu the ELF warnings are gone for me after applying this change.

Question: Should we do anything to prevent the subpackages to be installed at the same time?

Note: This doesn't totally fix running pmOS in qemu. The latter also needs crc32c_generic in its deviceinfo file but that seemed like a tangential fix so I split it off into !2694.

Fixes: #1294 (closed)