env: Switch env_nand, env_mmc and env_ubi to env_import_redund

The env_nand, env_mmc and env_ubi implementations all implement
redundancy using an identical serial-number scheme. This commit
migrates them to use the implementation in env_common, which is
functionally identical.

Signed-off-by: Fiach Antaw <fiach.antaw@uqconnect.edu.au>
Reviewed-by: Tom Rini <trini@konsulko.com>