When building only a bootstrap tarball, mkarchiso still validates packages.x86_64, bootmode and airootfs_image_type

The bootstrap build mode does not use packages.x86_64, ${bootmodes[@]} or $airootfs_image_type so there's no need to validate them.

Since we already have _validate_requirements_buildmode_bootstrap, _validate_requirements_buildmode_iso and _validate_requirements_buildmode_netboot, I think the package list, boot mode and airootfs image type checks should be moved from _validate_options to the respective _validate_requirements_buildmode_* functions.