hvmloader: add knob for fixed VGABIOS date string

hvmloader: add knob for fixed VGABIOS date string

To allow reproducible builds of hvmloader introduce a make variable
VGABIOS_REL_DATE="dd Mon 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>
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>
Acked-by: Ian Campbell <ian.campbell@citrix.com>