env: Add a command to view callbacks

The callbacks can be bound, but are otherwise invisible.  Add a command
to show what callbacks are available.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>

!!! fix callback command
diff --git a/README b/README
index e00a7c2..ac61a32 100644
--- a/README
+++ b/README
@@ -815,6 +815,7 @@
 		CONFIG_CMD_EDITENV	  edit env variable
 		CONFIG_CMD_EEPROM	* EEPROM read/write support
 		CONFIG_CMD_ELF		* bootelf, bootvx
+		CONFIG_CMD_ENV_CALLBACK	* display details about env callbacks
 		CONFIG_CMD_EXPORTENV	* export the environment
 		CONFIG_CMD_EXT2		* ext2 command support
 		CONFIG_CMD_EXT4		* ext4 command support