cli: allow verbatim character entry with CTRL-v

Up to now the U-Boot console does not allow to add a control character to
the line buffer. In Bash and Vim we can use CTRL-v for this purpose.

Add support for CTRL-v for verbatim entry of characters.

To keep things simple, while editing control characters are displayed as
simple characters, e.g. ESC (0x1b) is displayed as [ and not as ^[.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>