tty vt: Fix line garbage in virtual console on command line edition

On some machines using a specific hardware for console screen output,
the update of the pixel frame buffer does not work correctly when
using command line edition. This may be due to a memory cache bug
in the machine on which the problem has been found, but an other
solution is possible.

This patch proposes to avoid touching directly the pixel frame buffer
and to rebuild each character using the standard putc() function
on command line edition.

The resulting code is smaller and not obviously slower (no read
access to the pixel frame buffer).

Tested on a Cubox (ARM Marvell Dove 88AP510 SoC).

Signed-off-by: Jean-François Moine <moinejf@free.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>