scripts/extract-ikconfig: Support LZ4-compressed images.

Support for kernel image LZ4 compression was added around 3.11, but not
the corresponding kernel .config extraction.

This makes possible extracting the kernel config for LZ4-compressed
kernels you're not running, or the current LZ4-compressed kernel if
compiled without /proc/config.gz support.

Signed-off-by: Alex Pilon <alp+linux@alexpilon.ca>
Signed-off-by: Michal Marek <mmarek@suse.cz>