make kpatch-build a first class script

kpatch-build is outgrowing the kpatch script and probably is a better
fit as its own utility instead of being wrapped by kpatch.  Install
kpatch-build into /usr/local/bin, remove the kpatch wrapper around it,
and update the README accordingly.
kpatch-build --sourcedir option

Allow the user to supply a custom kernel source directory.  This copies
the directory to ~/.kpatch/src instead of using it in place.  Otherwise
the "make mrproper" (which is needed for compiling objects in a separate
tree) would modify the original source tree and remove its .config file.

There's currently no caching support for this option.  If needed, we
could implement that pretty easily by calculating an md5sum of the
original source directory.