dextrose-updater: fix yum check-update parsing

This solves the issue reported in [1], where only one
package appears in the logs, even if more packages were
updated.

1. https://olpcau.atlassian.net/browse/XOS-57

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Release dextrose-updater 5.102.2

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>