env: Add generic redundant environment implementation

All current environments that implement redundancy use almost
identical implementations. This patch implements the env_nand
implementation as a function in env_common, and updates the
env_export function to export an env_nand-style 'flags' field by
default.

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