env: add SAVEENV as an alias of the CMD_SAVEENV symbol

Currently, testing whether to compile in support for saving the
environment is a bit awkward when one needs to take SPL_SAVEENV into
account, and quite a few storage drivers currently do not honour
SPL_SAVEENV.

To make it a bit easier to decide whether environment saving should be
enabled, introduce SAVEENV as an alias for the CMD_SAVEENV
symbol. Then one can simply use

  CONFIG_IS_ENABLED(SAVEENV)
Signed-off-by: NRasmus Villemoes <rasmus.villemoes@prevas.dk>

env/Kconfig

@@ -3,6 +3,9 @@ menu "Environment"
 config ENV_SUPPORT
 	def_bool y
 
+config SAVEENV
+	def_bool y if CMD_SAVEENV
+
 config ENV_IS_NOWHERE
 	bool "Environment is not stored"
 	default y if !ENV_IS_IN_EEPROM && !ENV_IS_IN_EXT4 && \