Add 'editenv' command

The editenv command can be used to edit an environment variable.
Editing an environment variable is useful when one wants to tweak an
existing variable, for example fix a typo or change the baudrate in the
'bootargs' environment variable.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
diff --git a/include/config_cmd_default.h b/include/config_cmd_default.h
index a5d87a6..6e3903c 100644
--- a/include/config_cmd_default.h
+++ b/include/config_cmd_default.h
@@ -20,6 +20,7 @@
 #define CONFIG_CMD_BOOTD	/* bootd			*/
 #define CONFIG_CMD_CONSOLE	/* coninfo			*/
 #define CONFIG_CMD_ECHO		/* echo arguments		*/
+#define CONFIG_CMD_EDITENV	/* editenv			*/
 #define CONFIG_CMD_FPGA		/* FPGA configuration Support	*/
 #define CONFIG_CMD_IMI		/* iminfo			*/
 #define CONFIG_CMD_ITEST	/* Integer (and string) test	*/