Merge remote-tracking branch 'remotes/famz/tags/docker-pull-request' into staging

# gpg: Signature made Mon 04 Jun 2018 08:09:27 BST
# gpg:                using RSA key CA35624C6A9171C6
# gpg: Good signature from "Fam Zheng <famz@redhat.com>"
# Primary key fingerprint: 5003 7CB7 9706 0F76 F021  AD56 CA35 624C 6A91 71C6

* remotes/famz/tags/docker-pull-request:
  docker: Update fedora image to 28
  tests/docker/Makefile.include: handle empty TARGET_LIST

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>