Merge remote-tracking branch 'remotes/stsquad/tags/pull-travis-updates-010618-1'...

Some Travis & Docker updates

 - simplification of the build matrix
 - usability tweaks for docker
 - update Travis docker image

# gpg: Signature made Fri 01 Jun 2018 15:05:03 BST
# gpg:                using RSA key FBD0DB095A9E2A44
# gpg: Good signature from "Alex Bennée (Master Work Key) <alex.bennee@linaro.org>"
# Primary key fingerprint: 6685 AE99 E751 67BC AFC8  DF35 FBD0 DB09 5A9E 2A44

* remotes/stsquad/tags/pull-travis-updates-010618-1:
  .travis.yml: update GCC sanitizer build to GCC 7
  .travis.yml: make current setup explicit
  .travis.yml: rationalise clang testing
  docker: update Travis docker image
  docker: do not display deprecated images in 'make docker' help
  docker: sort images list displayed by 'make docker'
  .travis.yml: disable linux-user build for gcov

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