Build separate 64 bit images for Raspberry Pi 3

Created by: SunilMohanAdapa

Currently, we are using 32-bit Raspberry Pi 2 images for Raspberry 3 also. This is made possible because of backwards compatibility provided by Raspberry Pi 3.

However, Raspberry Pi 3 is a 64-bit processor and it would be better for performance if separate 64-bit images are built for Raspberry Pi 3.

In order to do this, create a separate target for Raspberry Pi3 in builder.py just like Raspberry Pi 2 but with arm64 architecture and arm64 kernel variant. After that in customization and hardware setup scripts, make sure to follow the same process for Raspberry Pi 2 and Raspberry Pi 3.