kexec/ppc64: move to device tree version 17

Kernel commit e6a6928c3ea1d0195ed75a091e345696b916c09b changed the way the
device tree is processed in the kernel. Now version 2 is no more supported.

This patch move the version of the device tree generated in ppc64
environment from 2 to 17, allowing to kexec kernel 3.16.

In addition, automates the define of NEED_STRUCTURE_BLOCK_EXTRA_PAD which
should not be set for DT version 16 and above.

Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: Simon Horman <horms@verge.net.au>