Merge remote-tracking branch 'remotes/stsquad/tags/pull-testing-fixes-100920-1' into staging

Various misc and testing fixes:

  - Expand CODING_STYLE.rst a little more
  - usb-host build fix
  - allow check-softfloat unit tests without TCG
  - simplify mips imm_branch so compiler isn't confused
  - mark ppc64abi32 for deprecation
  - more compiler soothing in pch_rev_id
  - allow acceptance to skip missing binaries
  - more a bunch of plugins to contrib

# gpg: Signature made Thu 10 Sep 2020 10:51:05 BST
# 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-fixes-100920-1:
  plugins: move the more involved plugins to contrib
  tests/acceptance: Add Test.fetch_asset(cancel_on_missing=True)
  tests: bump avocado version
  hw/i386: make explicit clearing of pch_rev_id
  configure: don't enable ppc64abi32-linux-user by default
  docs/system/deprecated: mark ppc64abi32-linux-user for deprecation
  target/mips: simplify gen_compute_imm_branch logic
  tests/meson.build: fp tests don't need CONFIG_TCG
  usb-host: restrict workaround to new libusb versions
  CODING_STYLE.rst: flesh out our naming conventions.

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