Fix building of rPi image

[tve]

This PR contains several separate commits that are necessary to perform a complete image build. Some of the issues resolved:

• use the correct dockcross version to end up with correctly built executables
• fix misc issues due to the current rpi OS being based on debian buster (most of these fixes are thanks to Evan Jobling)
• install node.js from nodesource to avoid node/npm mismatch (a bug in debian)
• fix various file locations, such as a default deployment.txt and new /etc/sensorgnome_id

Some issues remain:

• need to expand /data partition on first boot
• need to sort out custom openssh version
• make build script more incremental