This PR prevents older versions of deb packages from including systemd-unit
files.
Because of the way Krill is set up, this requires both Ubuntu 20.04 and
Debian 11 to have usrmerge (which both have by default, only if upgraded
from an older version it might be missing).
Cross compile DEB packages for armhf/ARMv7 and aarch64/ARM64 using the Rust Tools team cargo cross tool.
Tested on Raspbian 11/Debian Bullseye on a Raspberry Pi 4b (armhf) and Armbian/Debian Buster on a Rock64 (aarch64).
Doesn't yet support stripping of cross compiled binaries (revisit this when Cargo supports the Rust 1.58.0 strip feature) or automated determination of DEB dependencies for cross compiled binaries.
Also removes a duplicate `--system` argument to `useradd` in the RPM `postinst` script.
