Merge remote-tracking branch 'remotes/rth/tags/pull-dt-20180326' into staging

Fix a decodetree problem with 16-bit insns

# gpg: Signature made Mon 26 Mar 2018 15:35:04 BST
# 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-dt-20180326:
  scripts/decodetree: Fix insnmask not marked as global in main()

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