Provide gitignore and make clean rules for extra files to hide and clean up the
extra files produced by module signing stuff once it is added.  Also add a
clean up rule for the module content extractor program used to extract the data
to be signed.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>