Add support to domain builders for loading kernels with physical addresses in the...

Add support to domain builders for loading kernels with physical addresses in the elf paddr and entry fields.
Add a new __xen_guest header field to distinguish between kernels with
the previous use of the paddr field and the new use.
Add a new __xen_guest header field to control the kernel entry point,
since the elf header entry field now points to a physical address.
This header field is also useful for supporting alternative entry
points in kernel images which run both on xen and native.
Also add a kernel config option to control whether the resulting kernel
should include compatibility code to run on Xen 3.0.2 or whether such
code such be left out, resulting in a kernel which will only run on
newer Xen versions.  Default to having compatibility enabled.
Kernels built with the new use of the elf header fields would otherwise
not work on Xen versions prior to this changeset.

Signed-off-by: Christian Limpach <Christian.Limpach@cl.cam.ac.uk>