xen: do not build backends for targets that do not support xen

xen: do not build backends for targets that do not support xen

Change Makefile.objs to use CONFIG_XEN instead of CONFIG_XEN_BACKEND, so
that the Xen backends are only built for targets that support Xen.

Set CONFIG_XEN in the toplevel Makefile to ensure that files that are
built only once pick up Xen support properly.

Signed-off-by: Stefano Stabellini <[email protected]>
Tested-by: Greg Kurz <[email protected]>
Reviewed-by: Greg Kurz <[email protected]>
CC: [email protected]
CC: [email protected]
CC: [email protected]
CC: [email protected]
Message-Id: <1489694518[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>