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

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.