Firmware anti-rollback update

The latest firmwares from DJI introduces a little change to the anti-rollback protection.
Anti-rollback will prevent you from downgrading to an older range of firmwares.

So...if you experience that the firmware downgrade gets stuck around 15-20%, then its the anti-rollback that is doing its thing.

We will be releasing an update to NLD within the next 48 hours that will bypass the anti-rollback protection yet again.