selftests/kexec: update searching for the Kconfig

First check /lib/modules/`uname -r`/config, before using the IKCONFIG.
In addition, the configs.ko might be compressed.  Fix the configs.ko
name.

Reviewed-by: Petr Vorel <pvorel@suse.cz>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>