menuconfig: Fix rendering of long prefilled edit box string

The horizontal scroll (hscroll) wasn't initialized properly when the
initial (prefilled) contents of an edit box was longer than the edit box
itself (e.g. when saving with a long path in KCONFIG_CONFIG). Things
snapped back into place once a key was pressed.

Properly initialize hscroll to fix the initial rendering.

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>