Add -c|--config option to kpatch-build

In some circumstances, the kernel is built outside from the source tree,
thus we should specify the .config file of build path.

This patch changes the basic option parsing by using getopt, and add more
information in usage().

Signed-off-by: Jincheng Miao <jincheng.miao@gmail.com>
Merge pull request #77 from ryanmiao/master

Add -c|--config option to kpatch-build