{start,stop}-mycroft.sh: fix shellcheck issues
{start,stop}-mycroft.sh: fix shellcheck issues

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

MycroftAI/mycroft-core,MycroftAI/mycroft-core