core: never apply first boot presets in the initrd

core: never apply first boot presets in the initrd

Presets are useful to initialize uninitialized /etc, but that doesn't
apply to the initrd.

Also, let's rename etc_empty → first_boot. After all, the variable
doesn't actually reflect whether /etc is really empty, it just reflects
whether /etc/machine-id existed originally or not. Moreover, we later on
directly initialize manager_set_first_boot() from it, hence let's just
name it the same way all through the codepath, to make this all less
confusing.

See: #7100