xen/livepatch: simplify and unify logic in prepare_payload()

xen/livepatch: simplify and unify logic in prepare_payload()

The following sections: .note.gnu.build-id, .livepatch.xen_depends and
.livepatch.depends are mandatory and ensured to be present by
check_special_sections() before prepare_payload() is called.

Simplify the logic in prepare_payload() by introducing a generic function to
parse the sections that contain a buildid.  Note the function assumes the
buildid related section to always be present.

No functional change intended.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>