nixos/generic-extlinux-compatible: introduce boot.loader.generic-extlinux-compatible.populateCmd

This option exposes the builder command used to populate an image,
honoring all options except the -c <path-to-default-configuration>
argument.

Useful to have for sdImage.populateRootCommands.

Special care needs to be taken w.r.t cross - the populate command runs
on the host platform, the activation script on the build platform (so
the builders differ)