Add oslist generation for rpi-imager

This commit adds a new create-rpi-oslist script which automates the generation of an oslist file for the latest Apertis releases. It generates separate files for stable and development releases, which can be used standalone or combined through a master oslist file which should be uploaded separately (not generated as its contents are fixed).

This commit also includes a small patch to copy-release so create-rpi-oslist is automatically called when a new public image is released.

Signed-off-by: Arnaud Ferraris arnaud.ferraris@collabora.com