xen: allow architecture to choose how/whether to compress installed xen binary

xen: allow architecture to choose how/whether to compress installed xen binary

This is a follow up to "xen: arm: make zImage the default target which we
install".

On ARM the xen.gz binary installed into /boot is not immediately useful because
bootloaders (e.g. u-boot) do not unconditionally support decompression (except
via the uImage wrapper, which we currently do not support via our build system)

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>
Acked-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Jan Beulich <jbeulich@suse.com>