BeagleBoard: Added userbutton command

Based on commit f1099c7c43caf5bac3bf6a65aa266fade4747072
    Author: Greg Turner <gregturner@ti.com>
    Date:   Tue May 25 09:19:06 2010 -0500

    New u-boot command for status of USER button on BeagleBoard-xM

         Modified bootcmd to check the staus at boot time and set
	 filename of the boot script.

* Moved to a BeagleBoard specific file.
* Removed changes to default boot command from adding userbutton
  command.
* Made to handle pre-xM boards.
* Flipped polarity of the return value to avoid confusion.  Success (0)
  is when the button is pressed.  Failure (1) is when the button is NOT
  pressed.
* Used latest revision getting function.
* Used latest macros for board revision.
* Added xM-C revision definition (optional, since it was default)
* updated default configuration with UserButton functionality
  * Added a separate bootenv variable to load a user defined .txt file
  * Added an example, showing how a different environment file can be loaded with
    the user button pressed

Signed-off-by: Jason Kridner <jkridner@beagleboard.org>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
Signed-off-by: Joel A Fernandes <agnel.joel@gmail.com>
Cc: Greg Turner <gregturner@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>