bisector: refine -oexport-db
bisector: refine -oexport-db

bisector report -oexport-db now outputs an exekall ValueDB that only
contains the root values that were selected using the other options.

ARM-software/lisa,ARM-software/lisa,credp/lisa,credp/lisa,ARM-software/lisa,ARM-software/lisa,credp/lisa,credp/lisa