Leverage the bootspec `label` field in its intended way. The VERSION_ID
of the os-release in the stub now only contains the generation number
and the build time. This makes a correct PRETTY_NAME entirely dependent
on correct information in the bootspec `label` field.
Read the build time from generation symlinks in /nix/var/nix/profiles
instead of from the underlying derivation. The derivation build time
will always be a UNIX epoch of 0 because of the `nix-build` sandbox,
which is useless for identifying when a generation was created.
To minimize the number of arguments passed to `lzbt`, the loader config
is assembled outside `lzbt` and passed as a single argument.
Instead of reimplementing `consoleMode` under the `lanzaboote`
namespace, `config.loader.systemd-boot.consoleMode` is reused as is.
The process of installing systemd-boot is "smarter" because it now
considers a a few conditions instead of doing nothing if there is a file
at the deistination path. systemd-boot is now forcibly installed (i.e.
overwriting any file at the destination) if (1) there is no file at the
destination, OR (2) a newer version of systemd-boot is available, OR (3)
the signature of the file at the destination could not be verified.
Raito Bezarius