dracut.sh: check for logfile (--logfile option) and create it if necessary

If a logfile is passed to dracut via --logfile option and doesn't
exist, dracut doesn't create it and logs nothing. Instead, dracut
should try to touch the file and print a warning if creating fails.

References: bnc#892191
Merge branch 'sles12' of https://github.com/hreinecke/dracut into sles12