kexec: bump to version 2.0.2 + change URL to match kernel.org

Closes #3007

Changed the KEXEC_SITE to the new home of kexec. This patch also satisfies
bug #3007 created by me last month. Thanks to Gustavo for pointing out that
the easy fix was to bump kexec to 2.0.2.

Signed-of-by: Andy Kennedy <Andy.Kennedy@AdTran.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>