os-release: define /usr/lib/os-release as fallback for /etc/os-release

The file should have been in /usr/lib/ in the first place, since it
describes the OS container in /usr (and not the configuration in /etc),
hence, let's support os-release files in /usr/lib as fallback if no
version in /etc exists, following the usual override logic.

A prior commit already enabled tmpfiles to create /etc/os-release as a
symlink to /usr/lib/os-release should it be missing, thus providing nice
compatibility with applications only checking in /etc.

While it's probably a good idea if all apps check both locations via a
fallback logic, it is only necessary in the early boot process, as long
as the /etc/os-release symlink has not been restored, in case we boot
with an empty /etc.

src/core/dbus-manager.c

@@ -1191,7 +1191,7 @@ static int method_switch_root(sd_bus *bus, sd_bus_message *message, void *userda
         /* Safety check */
         if (isempty(init)) {
                 if (! path_is_os_tree(root))
-                        return sd_bus_error_setf(error, SD_BUS_ERROR_INVALID_ARGS, "Specified switch root path %s does not seem to be an OS tree. /etc/os-release is missing.", root);
+                        return sd_bus_error_setf(error, SD_BUS_ERROR_INVALID_ARGS, "Specified switch root path %s does not seem to be an OS tree. os-release file is missing.", root);
         } else {
                 _cleanup_free_ char *p = NULL;
 

src/core/main.c

@@ -1251,9 +1251,15 @@ static int status_welcome(void) {
                            "PRETTY_NAME", &pretty_name,
                            "ANSI_COLOR", &ansi_color,
                            NULL);
+        if (r == -ENOENT) {
+                r = parse_env_file("/usr/lib/os-release", NEWLINE,
+                                   "PRETTY_NAME", &pretty_name,
+                                   "ANSI_COLOR", &ansi_color,
+                                   NULL);
+        }
 
         if (r < 0 && r != -ENOENT)
-                log_warning("Failed to read /etc/os-release: %s", strerror(-r));
+                log_warning("Failed to read os-release file: %s", strerror(-r));
 
         return status_printf(NULL, false, false,
                              "\nWelcome to \x1B[%sm%s\x1B[0m!\n",