Merge remote-tracking branch 'remotes/xtensa/tags/20170606-xtensa' into staging

target/xtensa fixes:

- fix read/write simcall mapping flags and return value;
- use -serial option to direct console output of sim machine to QEMU chardev;
- fix handling of unknown registers in the gdbstub.

# gpg: Signature made Tue 06 Jun 2017 11:46:05 BST
# gpg:                using RSA key 0x51F9CC91F83FA044
# gpg: Good signature from "Max Filippov <filippov@cadence.com>"
# gpg:                 aka "Max Filippov <max.filippov@cogentembedded.com>"
# gpg:                 aka "Max Filippov <jcmvbkbc@gmail.com>"
# Primary key fingerprint: 2B67 854B 98E5 327D CDEB  17D8 51F9 CC91 F83F A044

* remotes/xtensa/tags/20170606-xtensa:
  target/xtensa: handle unknown registers in gdbstub
  target/xtensa: support output to chardev console
  target/xtensa: fix return value of read/write simcalls
  target/xtensa: fix mapping direction in read/write simcalls

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