Add MultiROM (or other method for dual boot) support

[nemanjan00]

This issue is still under construction.

This is draft from my memory, I need to do fact cheking.

I will try to reconstruct MultiROM, so, we can implement it easier.

MultiROM is dualboot script that runs after kernel and before init scripts.

It lets users pick different ROM (that are installed by TWRP fork).

It is using modified kexec-tools for running kernel of different ROM and also kexec-hardboot patch.

Before running another kernel, it mounts paths to proper locations, for that different ROM, from internal storage or USB flash (which seams like a better solution for us, right now, since we are using custom file storage)

Here, you can find list of modified kernels, to work with it.