xen: new package

xen: new package

Add support to compile Xen 4.6 and Xen tools for arm32 and arm64
targets.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
[Thomas:
 - Remove all "select" from the main Xen option, since the hypervisor
   can build fine without any additional library. The dependencies have
   been moved to the "tools" sub-option.
 - Make sure that at least one of the sub-option is enabled by selecting
   the hypervisor sub-option if the tools sub-option is not enabled.
 - Add a Config.in comment about the dependencies of the Xen tools.
 - Add missing dependency on argp-standalone in the .mk file.
 - Simplify the handling of XEN_INSTALL_TARGET_OPTS.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>