Merge remote-tracking branch 'remotes/cohuck/tags/s390x-20170815' into staging

Regression fix for 'make clean' on s390x.

# gpg: Signature made Tue 15 Aug 2017 08:24:54 BST
# gpg:                using RSA key 0xDECF6B93C6F02FAF
# gpg: Good signature from "Cornelia Huck <conny@cornelia-huck.de>"
# gpg:                 aka "Cornelia Huck <huckc@linux.vnet.ibm.com>"
# gpg:                 aka "Cornelia Huck <cornelia.huck@de.ibm.com>"
# gpg:                 aka "Cornelia Huck <cohuck@kernel.org>"
# gpg:                 aka "Cornelia Huck <cohuck@redhat.com>"
# Primary key fingerprint: C3D0 D66D C362 4FF6 A8C0  18CE DECF 6B93 C6F0 2FAF

* remotes/cohuck/tags/s390x-20170815:
  pc-bios/s390-ccw: Use rm command during make clean

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