Merge remote-tracking branch 'remotes/cohuck/tags/s390x-20170228' into staging

Network boot for s390x. More information (and instructions
for building a s390-netboot.img) can be found at
http://wiki.qemu-project.org/Features/S390xNetworkBoot

# gpg: Signature made Tue 28 Feb 2017 11:27:18 GMT
# gpg:                using RSA key 0xDECF6B93C6F02FAF
# gpg: Good signature from "Cornelia Huck <huckc@linux.vnet.ibm.com>"
# gpg:                 aka "Cornelia Huck <cornelia.huck@de.ibm.com>"
# Primary key fingerprint: C3D0 D66D C362 4FF6 A8C0  18CE DECF 6B93 C6F0 2FAF

* remotes/cohuck/tags/s390x-20170228:
  pc-bios/s390-ccw.img: rebuild image
  pc-bios/s390-ccw: Use the ccw bios to start the network boot
  s390x/ipl: Load network boot image
  s390x/ipl: Extend S390IPLState to support network boot
  elf-loader: Allow late loading of elf

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