[LIBS] vscreen: First-cut implementation of virtual screen binding

This patch adds initial implementation of virtual screen binding library.

The library in its current form only provides complete implementation of
vscreen_hard_bind(). The implementation of vscreen_soft_bind() is still
work-in-progress. We also have done limited testing of vscreen_hard_bind()
using "vscreen hard_bind" command on ARM fast models.

Signed-off-by: Anup Patel <anup@brainfault.org>