use more fixed strings to build the hypervisor

use more fixed strings to build the hypervisor

It should be possible to repeatedly build identical sources and get
identical binaries, even on different hosts at different build times.
This fails for xen.gz and xen.efi because current time and buildhost
get included in the binaries.

Provide variables XEN_BUILD_DATE, XEN_BUILD_TIME and XEN_BUILD_HOST
which the build environment can set to fixed strings to get a
reproducible build.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>