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

src/core/manager.c

@@ -1345,8 +1345,9 @@ int manager_startup(Manager *m, FILE *serialization, FDSet *fds) {
         if (r < 0)
                 return r;
 
+        /* If this is the first boot, and we are in the host system, then preset everything */
         if (m->first_boot > 0 &&
-            m->unit_file_scope == UNIT_FILE_SYSTEM &&
+            MANAGER_IS_SYSTEM(m) &&
             !m->test_run_flags) {
 
                 r = unit_file_preset_all(UNIT_FILE_SYSTEM, 0, NULL, UNIT_FILE_PRESET_ENABLE_ONLY, NULL, 0);