am335x, guardian: load env from NAND

- As there is a requirement to store certain data, we need a persistent
    storage in u-boot. Hence, we need to save env in NAND

  - Add default Guardian environment variables

  - Update partition table:
    - Reserve some space for experimentation, this ensures proper
      backwards compatibility

  - Update defconfig accordingly

Signed-off-by: Moses Christopher <BollavarapuMoses.Christopher@in.bosch.com>