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>

include/configs/omap3_beagle.h

@@ -226,7 +226,8 @@
 		"omapdss.def_disp=${defaultdisplay} " \
 		"root=${nandroot} " \
 		"rootfstype=${nandrootfstype}\0" \
-	"loadbootenv=fatload mmc ${mmcdev} ${loadaddr} uEnv.txt\0" \
+	"bootenv=uEnv.txt\0" \
+	"loadbootenv=fatload mmc ${mmcdev} ${loadaddr} ${bootenv}\0" \
 	"importbootenv=echo Importing environment from mmc ...; " \
 		"env import -t $loadaddr $filesize\0" \
 	"loaduimage=fatload mmc ${mmcdev} ${loadaddr} uImage\0" \
@@ -240,8 +241,12 @@
 
 #define CONFIG_BOOTCOMMAND \
 	"if mmc rescan ${mmcdev}; then " \
+		"if userbutton; then " \
+			"setenv bootenv user.txt;" \
+		"fi;" \
 		"echo SD/MMC found on device ${mmcdev};" \
 		"if run loadbootenv; then " \
+			"echo Loaded environment from ${bootenv};" \
 			"run importbootenv;" \
 		"fi;" \
 		"if test -n $uenvcmd; then " \