Fix AppStream metadata issues

Fix various issues reported by appstreamcli validate, which in turn resulted in lintian 'appstream-metadata-validation-failed' warnings.

• d/b/gencontrol.py: Change template substitution to allow for Jinja2 templates
• d/templates: Convert metainfo.xml template to Jinja2 format
• Fix AppStream metadata issues:
  • Remove 'package' from component IDs so they are <= 40 chars long
  • Move short description to <summary> and long description to <description>
  • Add homepage URL
  • Change filenames to match component IDs

(This is basically the same changes as firmware-nonfree!123 (merged) but in firmware-free.)