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: Rasmus Villemoes <rasmus.villemoes@prevas.dk>