hvmloader: add knob for fixed SMBIOS date string

hvmloader: add knob for fixed SMBIOS date string

To allow reproducible builds of hvmloader introduce a make variable
SMBIOS_REL_DATE=mm/dd/yyyy to provide a fixed date string. Without this
change the hvmloader binary changes with every rebuild.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Jan Beulich <jbeulich@suse.com>
Cc: Keir Fraser <keir@xen.org>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>