This supplementary target allows to run appstream-generator on the port's
STAGEDIR to see if its AppStream metadata is correct. It requires the
appstream-generator package to be installed. The asgen-config.json file is
generated from a template and is filled with stubs.
To properly generate the AppStream metadata Poudriere is required.
This is a supplemental change for https://github.com/freebsd/poudriere/pull/1237
which describes why do we need AppStream at all.