Change raspberrypi3 to 64-bit

The Yocto community started supporting Raspberrypi 3 by basically building the same code as for Raspberrypi 2 (32 bit) and GDP did the same.  The community has since then developed more support for the 64-bit code that the R-Pi3 also supports.

Khem Raj's blog post could be a starting point.
http://himvis.com/bake-64-bit-raspberrypi3-images-with-yoctoopenembedded/

Basically it should boil down to setting the target ($MACHINE) to raspberrypi3-64 instead of raspberrypi3

Tasks

• Change target of R-Pi 3 to build 64-bit code, and remove the 32-bit version.
• (Keep the 32-bit build for R-Pi 2)
• Adjust what's needed to make everything work, including Qt graphics, etc.

Acceptance Criteria

• **Tested with equivalent function as before
• Merged to master
• Old 32-bit is removed/replaced for R-Pi 3 (kept for R-Pi 2).