Update refactor

[TrystanLea]

• Refactor of update scripts
• Part way to full multi environment update process (compatible with /usr/emoncms install location)
• Modularised service install and update
• Improved image version check for future updates
• Updates can be run from command line, see emonpi/update/readme.md
• emonSD_install_test script for image build

[borpin]

https://github.com/openenergymonitor/emonpi/blob/83a80f588c24dd4a219c83166e4793efd4624369/install/emonSD_build_test.sh#L34-L34

I really do not like installing everything in one big chunk.

I'd rather this was split up so that the packages were installed and configured as they go along.

This would then aid things like not using apache.

UFW is installed but never used (for instance).