MODSIGN: Provide gitignore and make clean rules for extra files

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: David Howells <dhowells@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>