kpatch-build: --sourcedir 2.0

In my experience this is a much more useful implementation of the
"--sourcedir" option:

- use the source tree in-place rather than first copying it to
  ~/.kpatch/src.  In my case this avoids a 5GB copy, including the
  entire .git subdirectory, and allows ccache to be reused.

- find the vmlinux and .config files in the sourcedir

- autodetect the ARCHVERSION
fix review comment
Merge pull request #258 from jpoimboe/sourcedir-2.0

kpatch-build: --sourcedir 2.0