Merge remote-tracking branch 'remotes/stsquad/tags/pull-testing-040220-1' into staging

Merge remote-tracking branch 'remotes/stsquad/tags/pull-testing-040220-1' into staging

Testing updates and build fixes:

  - move more cross compilers to buster
  - fix build breakage (hppa Kconfig)
  - disable docs on shippable
  - build docs under bionic with python3
  - travis.yml re-factoring
  - check capabilities of non-docker compilers
  - smarter make -j parallelism

# gpg: Signature made Tue 04 Feb 2020 17:16:40 GMT
# gpg:                using RSA key 6685AE99E75167BCAFC8DF35FBD0DB095A9E2A44
# gpg: Good signature from "Alex Bennée (Master Work Key) <alex.bennee@linaro.org>" [full]
# Primary key fingerprint: 6685 AE99 E751 67BC AFC8  DF35 FBD0 DB09 5A9E 2A44

* remotes/stsquad/tags/pull-testing-040220-1:
  .travis.yml: ensure python3-sphinx installed for docs
  .travis.yml: single thread build-tcg
  .travis.yml: drop cris-linux-user from the plugins test
  .travis.yml: drop the travis_retry from tests
  .travis.yml: introduce TEST_BUILD_CMD and use it for check-tcg
  tests/tcg: gate pauth-% tests on having compiler support
  tests/tcg: add a configure compiler check for ARMv8.1 and SVE
  .travis.yml: probe for number of available processors
  .travis.yml: move cache flushing to early common phase
  .travis.yml: build documents under bionic
  .travis.yml: Add description to each job
  .travis.yml: Drop superfluous use of --python=python3 parameter
  .shippable: --disable-docs for cross-compile tests
  travis.yml: Install genisoimage package
  tests/docker: better handle symlinked libs
  tests/docker: move most cross compilers to buster base

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