Split licence out of sdcard.img

Resolves issue #1 (closed)

Tested on the example app: https://gitlab.com/celduin/bsps/example-app/-/tree/dwinship/extract-licence

Output https://celduin-cache.codethink.co.uk:8080/dev/bsp_poc/dwinship/extract-licence/pi/sdcard-image-and-licenses.tar (That's a 2GB download, contains sdcard.img and a separate licenses folder. I forgot to compress it.)