mediatek:mt7986: add default environment, change bl2 to atf
@AGnau: Add default environment for mt7986. Change partition name from bl2 to atf. Run mtd_probe_devices() to set the environment variable mtdparts when using the command iopsys_upgrade.
@AGnau: Add default environment for mt7986. Change partition name from bl2 to atf. Run mtd_probe_devices() to set the environment variable mtdparts when using the command iopsys_upgrade.