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/kernel-install/90-loaderentry.install

@@ -32,6 +32,8 @@ fi
 
 if [[ -f /etc/os-release ]]; then
     . /etc/os-release
+elif [[ -f /usr/lib/os-release ]]; then
+    . /usr/lib/os-release
 fi
 
 if ! [[ $PRETTY_NAME ]]; then