Fix MTK BL2 upgrade error

Fix bootloader update on MTK devices

Kept commented out BL2 erase as BL2 is marked read-only in dts. Erase on read-only mtd will return an error and stop upgrade. RO flag on BL2 has a good reason - corrupted BL2 will brick a device (with no u-boot). Leave BL2 untouched until figure out a better solution.

We may improve this with checking RO flag at __write_fit_image_component_to_mtd Removing RO flag needs some discussion due to possible problems.