Merge remote-tracking branch 'remotes/stsquad/tags/pull-shippable-disable-ppc-080318...

One fix to disable broken ppc cross-compile test on shippable

# gpg: Signature made Thu 08 Mar 2018 15:00:23 GMT
# 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-shippable-disable-ppc-080318-1:
  .shippable.yml: disable powerpc-cross image

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