Tidy and document

• Create build command so build doesn't start if a different command is entered incorrectly
• Add help message for build.sh
• Remove unused functions.sh
• Fix issue where python client env is written to /application not /var/openflexure/application due to an incorrect variable name
• Remove non standard backticks and replace with $()
• Remove deprecated $[] and replace with POSIX standard $(())
• Remove the null string hack for comments and replace with standard comments.
• Fix other issues found by schellcheck
• Replace multiline here-documents with copying or catting files from within the /step/ directory

This has been used to build an image using the stripped down image branch, but hasn't been tested on a microscope.

Things to do before merging:

• Re-enable variable substitution in multiline heredocs that have been moved to files.
• Test on a microscope that the server runs and is configured correctly.

Closes #16 (closed)