Merge remote-tracking branch 'remotes/stsquad/tags/pull-docker-updates-050618-1'...

Docker related updates

  - configure/probe for docker
  - TARGET_DIRS -> TARGET_LIST cleanup
  - new fedora-i386-cross and debian/tricore images
  - test-mingw use SDL2/GTK3

# gpg: Signature made Tue 05 Jun 2018 16:32:53 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-docker-updates-050618-1:
  docker: add debian/tricore image
  docker: test-mingw: use SDL2 and GTK+3
  docker: Add fedora-i386-cross image
  Makefile: Rename TARGET_DIRS to TARGET_LIST
  configure: add test for docker availability
  docker: add "probe" command for configure

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