i2c-tools: use upstream tarball

Tarballs of the releases are now again available:

https://www.spinics.net/lists/linux-i2c/msg30349.html



So change back to that instead of getting the source code from git.

While we're at it, add a hash for the license file.

[Peter: Also update Config.in homepage URL as pointed out by Baruch]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>