Prefer vt(4) for UEFI boot

The UEFI framebuffer driver vt_efifb requires vt(4), so add a mechanism
for the startup routine to set the preferred console.  This change is
ugly because console init happens very early in the boot, making a
cleaner interface difficult.  This change is intended only to facilitate
the sc(4) / vt(4) transition, and can be reverted once vt(4) is the
default.