sd-boot: allow configuration of log levels (#38701)

This allows for more liberal usage of logging functionality as messages
will no longer always show up on screen, regardless of urgency. The log
level to use can be configured through an SMBIOS type 11 string
(`io.systemd.boot.loglevel=`) or by using the `log-level` option in
loader.conf. Valid values are debug, info, notice, warning, err, crit,
alert, and emerg. By default, info will be used.