Crossgrading from armhf to arm64

[mad-ady]

First of all - thanks for your work. It has helped me to crossgrade from a armhf to a arm64 board. :)

I've made a fork with some changes (mostly I changed i386 to armhf and amd64 to arm64), but sadly I can't send a patch to you because my changes were in-place. Maybe there is a way to add/detect the architecture and handle the crossgrading more easily. Currently the architecture is hardcoded in several command lines and replacing it with variables requires rewriting may of those lines.

Here is my fork, for reference: https://github.com/mad-ady/distrorejuve, and here is documentation about how the process went: https://forum.odroid.com/viewtopic.php?f=55&t=33861

I hope arm can be integrated in your tool in an elegant way :)