add menutree command ...

Creating menus from the shell using the regular 'menu' command is rather
complicated. This adds a 'menutree' command which creates a menu from
a directory structure. In the directory structure each directory corresponds
to a single menu entry. The directory contains the following files:

title -  A file containing the title of the entry as shown in the menu
box -    If present, the entry is a 'bool' entry. The file contains a variable
         name from which the current state of the bool is taken from and saved
         to.
action - if present this file contains a shell script which is executed when
         when the entry is selected.

If neither 'box' or 'action' are present this entry is considered a submenu
containing more entries.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>