ShellCheck is a static analysis tool for shell scripts with the goal to:
- point out and clarify typical beginner's syntax issues that cause a
shell to give cryptic error messages
- point out and clarify typical intermediate level semantic problems
that cause a shell to behave strangely and counter-intuitively
- point out subtle caveats, corner cases and pitfalls that may cause an
advanced user's otherwise working script to fail under future
circumstances

I've ran this tool over both start-mycroft.sh and stop-mycroft.sh and
fixed any issue that popped up

This makes the start and stop scripts compatible with POSIX shells.

Overview of the changes:
- "function" statements removed, not necessary and incompatible
- dashes in function and variable names for lower ones (- to _)
- source statements changed for .
- double square brackets replaced for single ones
- double equal statements replaced for single ones
- &> (piping stdout and stderr to the same file) replaced for 2>&1 >
- sourcing of mycroft-skill-testrunner replaced with direct execution
with Bash
- replaced BASH_SOURCE with $0, these scripts are never sourced anyway
- replaced "echo -n" statements with "printf"
- merged the "" and "all" cases to a single one