Merge remote-tracking branch 'remotes/rth/tags/pull-decode-20180222' into staging

Add decodetree.py

# gpg: Signature made Thu 22 Feb 2018 23:44:43 GMT
# gpg:                using RSA key 64DF38E8AF7E215F
# gpg: Good signature from "Richard Henderson <richard.henderson@linaro.org>"
# Primary key fingerprint: 7A48 1E78 868B 4DB6 A85A  05C0 64DF 38E8 AF7E 215F

* remotes/rth/tags/pull-decode-20180222:
  scripts: Add decodetree.py

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