main: change check whether /etc is unpopulated to look for /etc/machine-id

Previously, we checked whether /etc was completely empty. This makes it
difficult though for container managers such as nspawn to install a
small number of files (such as /etc/timezone), and have the system
otherwise populate its own tree.

Hence, change this by looking for /etc/machine-id, which should be a
good sign whether /etc is populated or not.

src/core/main.c

@@ -1543,7 +1543,7 @@ int main(int argc, char *argv[]) {
                 if (in_initrd())
                         log_info("Running in initial RAM disk.");
 
-                empty_etc = dir_is_empty("/etc") > 0;
+                empty_etc = access("/etc/machine-id", F_OK) < 0;
                 if (empty_etc)
                         log_info("Running with unpopulated /etc.");
         } else {