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>