firstboot: add debug logging

firstboot: add debug logging

$ SYSTEMD_LOG_LEVEL=debug build/systemd-firstboot --prompt-root-password
Found container virtualization systemd-nspawn.
Found /etc/locale.conf, assuming locale information has been configured.
Failed to read credential firstboot.keymap, ignoring: No such device or address
Prompting for keymap was not requested.
Found /etc/localtime, assuming timezone has been configured.
Prompting for hostname was not requested.
Found /etc/machine-id, assuming machine-id has been configured.
Found /etc/passwd and /etc/shadow, assuming root account has been initialized.
Creation of /etc/kernel/cmdline was not requested, skipping.