(Urgent) Use HTTPS instead of HTTP when sites have valid mainstream certs

(Make the build more resistant against MITM attacks, part #1)

http://rcn-ee.net -> https://rcn-ee.net
http://github.com -> https://github.com
http://www.debian.org -> https://www.debian.org

This patch also applies to https://github.com/beagleboard/image-builder - should I submit a separate pull request for that?