Merge remote-tracking branch...

Merge remote-tracking branch 'remotes/stsquad/tags/pull-code-coverage-and-build-tweaks-050718-3' into staging

Code coverage and other build tweaks

  - revert 208ecb3e (and drop filter for mingw, tweak for check-tcg)
  - some travis speed-ups
  - modernise code coverage support
  - docker image cleanups
  - clean-up binfmt_misc docker infrastructure
  - add debian-powerpc-user-cross image for ppc32 build

# gpg: Signature made Thu 05 Jul 2018 17:00:02 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-code-coverage-and-build-tweaks-050718-3:
  docker: add linux-user powered cross builder for QEMU
  docker: add special rule for deboostrapped images
  docker: add special handling for FROM:debian-%-user targets
  docker: debian-bootstrap.pre allow customising of variant/url
  docker: drop QEMU build-dep from bootstrap
  docker: Do not run tests in 'intermediate' images
  docker: Clean the MXE base image
  docker: ubuntu: Use SDL2
  docker: ubuntu: Update the package list before installing new ones
  linux-user: add gcov support to preexit_cleanup
  linux-user: introduce preexit_cleanup
  build-system: add coverage-report target
  build-system: add clean-coverage target
  travis: add gcovr summary for GCOV build
  docker: add gcovr to travis image
  .gitignore: add .gcov files
  build-system: remove per-test GCOV reporting
  travis: test out-of-tree builds
  travis: do not waste time cloning unused submodules
  Revert "Makefile: Rename TARGET_DIRS to TARGET_LIST"

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