kconfig: have extract-ikconfig read ELF files

It would be nice to use extract-ikconfig to find the congfig.gz
in either vmlinux (not vmlinuz) or configs.ko.

This patch changes the script to also be able to read ELF files directly.

[ Impact: find config.gz in vmlinux and configs.ko ]

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>