Download packages from github in tar.gz format instead git format.

Download packages from github in tar.gz format instead git format.

Verified way how to get packages from github by version or sha1
(it's enough to use first 7 characters from SHA1):
1. by sha1
FOO_VERSION = 1234567
FOO_SITE = http://github.com/user/package/tarball/branch

2. by version
FOO_VERSION = v1.0
FOO_SITE = http://github.com/user/package/tarball/$(FOO_VERSION)

Signed-off-by: Marek Belisko <marek.belisko@open-nandra.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>