os-release: add 'SYSEXT_' fields for version/id

sysext DDI cannot carry an os-release file, but have to carry
an extension-release file. But so far, this was only used to
match the sysext DDI with the base DDI/rootdir. It is also
useful to describe the sysext DDI itself, just like we do in
os-release.

So document that the same fields used in os-release can also
be added to an extension-release, with the 'SYSEXT_' prefix,
and in that case they are understood to define the sysext DDI
itself, rather than for matching purposes.

diff --git a/man/os-release.xml b/man/os-release.xml
index 113ef9fc18a8f0fc038862faa17882de9994f9aa..e74f27b9907e42fc2adb0d80f05be08708d9d1cc 100644 (file)
--- a/man/os-release.xml
+++ b/man/os-release.xml
@@ -111,6 +111,11 @@
       <varname>VERSION_ID=</varname> exists and matches. This ensures ABI/API compatibility between the
       layers and prevents merging of an incompatible image in an overlay.</para>
 
+      <para>In order to identify the extension image itself, the same fields defined below can be added to the
+      <filename>extension-release</filename> file with a <varname>SYSEXT_</varname> prefix (to disambiguate
+      from fields used to match on the base image). E.g.: <varname>SYSEXT_ID=myext</varname>,
+      <varname>SYSEXT_VERSION_ID=1.2.3</varname>.</para>
+
       <para>In the <filename>extension-release.<replaceable>IMAGE</replaceable></filename> filename, the
       <replaceable>IMAGE</replaceable> part must exactly match the file name of the containing image with the
       suffix removed. In case it is not possible to guarantee that an image file name is stable and doesn't