Support for building in a Xen binary

Support for building in a Xen binary

Add support for building a Xen binary which includes a Dom0 image and
the Dom0 command-line.

If the user specifies --with-xen=<Xen>, where Xen is an appropriate
AArch64 Xen binary, the build system will generate a xen-system.axf
instead of a linux-system.axf.

Original patch from Ian Campbell, but I modified most of it so all bugs
are probably mine.
[Andre: adapt to newest boot-wrapper branch, increase load address]

Cc: Ian Campbell <ijc@hellion.org.uk>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>