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/README b/README
index dee0e67..2c77687 100644
--- a/README
+++ b/README
@@ -620,6 +620,7 @@
 		CONFIG_CMD_DS4510_RST	* ds4510 I2C rst command
 		CONFIG_CMD_DTT		* Digital Therm and Thermostat
 		CONFIG_CMD_ECHO		  echo arguments
+		CONFIG_CMD_EDITENV	  edit env variable
 		CONFIG_CMD_EEPROM	* EEPROM read/write support
 		CONFIG_CMD_ELF		* bootelf, bootvx
 		CONFIG_CMD_SAVEENV	  saveenv