xen: infrastructure to have cross-platform video drivers

xen: infrastructure to have cross-platform video drivers

- introduce a new HAS_VIDEO config variable;
- build xen/drivers/video/font* if HAS_VIDEO;
- rename vga_puts to video_puts;
- rename vga_init to video_init;
- rename vga_endboot to video_endboot.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>
Committed-by: Ian Campbell <ian.campbell@citrix.com>