Reproducible builds

Mangos builds should be reproducible, i.e. the output should be byte-for-byte identical when built with the same config, whether on the same build system or on another.

Known differences:
 * `root` and `root-verity` (not `root-verity-sig`) partition UUID's differ, even with a consistent seed passed to sd-repart. 
 * Timestamp in secure boot signatures differ.