fix building when saveenv is disabled in some setups

If you enable environment in the flash, but disable the embedded
option, and you disable the saveenv command, then the #if nested
logic will trigger a compile failure:
env_flash.c: In function 'env_relocate_spec':
env_flash.c:399: error: 'flash_addr' undeclared (first use in this function)
The fix is to add CMD_SAVEENV ifdef protection like everywhere else.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>