Merge remote-tracking branch 'remotes/rth/tags/pull-dt-20190506' into staging

Add support for variable-length ISAs

# gpg: Signature made Mon 06 May 2019 19:26:21 BST
# gpg:                using RSA key 7A481E78868B4DB6A85A05C064DF38E8AF7E215F
# gpg:                issuer "richard.henderson@linaro.org"
# gpg: Good signature from "Richard Henderson <richard.henderson@linaro.org>" [full]
# Primary key fingerprint: 7A48 1E78 868B 4DB6 A85A  05C0 64DF 38E8 AF7E 215F

* remotes/rth/tags/pull-dt-20190506:
  decodetree: Add DisasContext argument to !function expanders
  decodetree: Expand a decode_load function
  decodetree: Initial support for variable-length ISAs

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