menuconfig:inputbox: support navigate input position

This patch add support navigate input position *inside* the input
field with LEFT/RIGHT, so it is possible to modify the text in place.

Signed-off-by: Wang YanQing <udknight@gmail.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>