Merge remote-tracking branch 'remotes/artyom/tags/pull-sun4v-20170226' into staging

Pull request for Niagara patches 2017 02 26

# gpg: Signature made Sun 26 Feb 2017 21:56:06 GMT
# gpg:                using RSA key 0x3360C3F7411A125F
# gpg: Good signature from "Artyom Tarasenko <atar4qemu@gmail.com>"
# gpg: WARNING: This key is not certified with a trusted signature!
# gpg:          There is no indication that the signature belongs to the owner.
# Primary key fingerprint: 2AD8 6149 17F4 B2D7 05C0  BB12 3360 C3F7 411A 125F

* remotes/artyom/tags/pull-sun4v-20170226:
  niagara: check if a serial port is available
  niagara: fail if a firmware file is missing

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