[console] Fix display of characters with top bit set

Inhibit implicit sign-padding of characters with the top bit set
(e.g. accented characters), which confuses the mucurses library by
colliding with the bits used to store character attributes and
colours.

Reported-by: Marc Delisle <Marc.Delisle@cegepsherbrooke.qc.ca>
Signed-off-by: Michael Brown <mcb30@ipxe.org>