systemd's meson.build looks up various systems utilties at build-time. This wasn't a problem before, but as our buildroots are now usr-merged we can no longer expect that meson's $PATH traversal results in a path to a binary on non-usr-merge system. We need to state our expectations explictly now. This also removes some unneeded options. meson's get_option returns on booleans without any default `true`. Part-of: https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/6884 [ci:skip-build]: already built successfully in CI