env: Introduce support for MTD

env: Introduce support for MTD

Introduce support for env in generic MTD. Currently we only support SPI
flash based on the lagacy sf cmd that assume SPI flash are always NOR.
This is not the case as to SPI controller also NAND can be attached.

To support also these flash scenario, add support for storing and
reading ENV from generic MTD device by adding an env driver that
base entirely on the MTD api.

Introduce a new kconfig CONFIG_ENV_IS_IN_MTD and
CONFIG_ENV_MTD_DEV to define the name of the MTD device as exposed
by mtd list.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>