merge_config.sh: improve indentation

It is true that we do not want to move the code too far to the
right, but something like below is not preferred:

    if [ "x$PREV_VAL" != "x$NEW_VAL" ] ; then
    echo Value of $CFG is redefined by fragment $MERGE_FILE:
    echo Previous  value: $PREV_VAL
    echo New value:       $NEW_VAL
    echo
    elif [ "$WARNREDUN" = "true" ]; then
    echo Value of $CFG is redundant by fragment $MERGE_FILE:
    fi

To fix this, call "continue" if the "grep" command fails to find the
given CONFIG.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>