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