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>
menu: export menu_default_choice() function

Checking the default menu item and obtaining its data can
be useful in custom menu code. Export menu_default_choice()
function which serves this purpose.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
New command bootmenu: ANSI terminal boot menu support

The "bootmenu" command uses U-Boot menu interfaces and provides
a simple mechanism for creating menus with several boot items.
When running this command the menu will be assembled as defined
by a set of environment variables which contain a title and
command key-value pairs. The "Up" and "Down" keys are used for
navigation through the items. Current active menu item is
highlighted and can be selected using the "Enter" key.

The command interprets and generates various ANSI escape
sequencies, so for proper menu rendering and item selection
the used terminal should support them.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
[agust: various fixes and documentation updates]
Signed-off-by: Anatolij Gustschin <agust@denx.de>
RX-51: Add support for bootmenu

 * default bootmenu entries:
   attached kernel, internal eMMC memory, external SD card,
   u-boot boot order

 * in CONFIG_PREBOOT try load bootmenu.scr from first partition
   of internal eMMC memory (also known as MyDocs) which (should)
   overwrite default bootmenu entries

 * when keyboard slide is closed boot first menu entry

 * when keyborad slide is open show bootmenu

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Merge branch 'agust@denx.de' of git://git.denx.de/u-boot-staging