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/bootchart/bootchart.c

@@ -386,10 +386,10 @@ int main(int argc, char *argv[]) {
                 if (sysfd < 0)
                         sysfd = open("/sys", O_RDONLY|O_CLOEXEC);
 
-                if (!build)
-                        parse_env_file("/etc/os-release", NEWLINE,
-                                       "PRETTY_NAME", &build,
-                                       NULL);
+                if (!build) {
+                        if (parse_env_file("/etc/os-release", NEWLINE, "PRETTY_NAME", &build, NULL) == -ENOENT)
+                                parse_env_file("/usr/lib/os-release", NEWLINE, "PRETTY_NAME", &build, NULL);
+                }
 
                 /* wait for /proc to become available, discarding samples */
                 if (graph_start <= 0.0)