core: make `/run/systemd/first-boot` available earlier

core: make `/run/systemd/first-boot` available earlier

For the new systemd.credentials_boot_policy= setting we need
to be able to differenciate if its a firstboot or not early.

But `manager_set_first_boot` is run after `initialize_runtime()`
which imports the credentials. So we need to touch the file
so that the credentials_boot_policy setting can work.

Unfortunately we cannot just move manager_set_first_boot() earlier
because there is no manager yet at this point.