u-boot support, and /boot on root support

[gutschke]

I was trying to use your scripts on one of my devices that uses Das U-Boot to load Armbian. Interestingly enough, it also doesn't have a dedicated /boot partition, but just uses a regular folder in the root filesystem. This means, you can't actually access /boot and make it r/w from within the overlay filesystem.

Added support to make all of that work. I don't know whether it works for other Das U-Boot systems, nor whether it works for Armbian systems that use a different way of booting. But I tried to keep things as generic as I could, so it shouldn't break anything and it stands at least a fighting chance to work on other systems.