docs/support-matrix: unbreak docs rendering

docs/support-matrix: unbreak docs rendering

The cronjob which renders https://xenbits.xen.org/docs/ has been broken for a
while.  commitish_version() pulls an old version of xen/Makefile out of
history, and uses the xenversion rule.

Currently, this fails with:

  tmp.support-matrix.xen.make:130: scripts/Kbuild.include: No such file or directory

which is because the Makefile legitimately references Kbuild.include with a
relative rather than absolute path.

Rework support-matrix-generate to use sed to extract the major/minor version,
rather than expecting xen/Makefile to be usable in a different tree.

Fixes: 945e80a7301f ("docs: Provide support-matrix-generate, to generate a support matrix in HTML")
Backport: 4.11+
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Paul Durrant <paul@xen.org>