Fix bogus variable reference in mkimage-yum.sh

Fixes this: https://github.com/dotcloud/docker/commit/d419da7227826e84e9375ece4fd9d4978a42cbf7#commitcomment-5344982

Docker-DCO-1.1-Signed-off-by: Chris St. Pierre <chris.a.st.pierre@gmail.com> (github: stpierre)
Merge pull request #4093 from stpierre/mkimage-yum

Fix bogus variable reference in mkimage-yum.sh