[C72n][ENV] fix default root device id
By default, env variable root_device_id is 1 (uSD) while fdt_device_id, kernel_device_id, ramfs_device_id, etc are 0 (MMC). Set all to 0 for c72n
By default, env variable root_device_id is 1 (uSD) while fdt_device_id, kernel_device_id, ramfs_device_id, etc are 0 (MMC). Set all to 0 for c72n