menu: Add support for user defined item choice function

Selecting menu items is currently done in menu_interactive_choice()
by reading the user input strings from standard input.

Extend menu_interactive_choice() to support user defined function
for selecting menu items. This function and its argument can be
specified when creating the menu.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Anatolij Gustschin <agust@denx.de>