archiso/mkarchiso: Generalize handling of ucodes

archiso/mkarchiso: Generalize the handling of ucode files (e.g. copying, signing, size calculation) by introducing the global readonly array of expected ucode names instead of hardcoding them in various functions. When making use of ucode files in _make_boot_on_iso9660(), _make_bootmode_uef-x86.systemd-boot.esp() and _sign_netboot_artifacts() create a list of files that is guaranteed to contain existing files.

Fixes #132 (closed)