Make use of shellcheck

[Vogtinator]

https://www.shellcheck.net checks bash (and other shell) scripts for errors and possible bugs, we could make use of that for CI.
This would probably require quite a few fixes and shellcheck ignores to make it really useful though.
Needs some experimenting.

[gmoro]

I tried that before, but had no time to really put it to use, would be a great addition

[Vogtinator]

We should try to use relative source paths for inclusion to fix issues like

In files/usr/sbin/jeos-config line 23:
. "/usr/share/jeos-firstboot/jeos-firstboot-functions"
  ^-- SC1091 (info): Not following: /usr/share/jeos-firstboot/jeos-firstboot-functions was not specified as input (see shellcheck -x).

e.g. in jeos-firstboot and jeos-config do cd "$(dirname $0)" to ensure we're in the right location and then use . ../share/jeos-firstboot/.... That would also make it possible to test without installing it system-wide.