Add shellcheck action, fix issues

[matthiasbeyer]

This PR adds a shellcheck job. Shellcheck is reasonably fast that this does not impact our CI time at all.

This PR also fixes most issues shellcheck reports. The ones I did not fix here are issues where I don't know how to go forward and might need some investigation from someone who knows better.

[matthiasbeyer]

Ah, we should probably fix the remaining ones in this PR as well.

Right now there are 23 issues left that we need to fix before the job succeeds.

[matthiasbeyer]

That does mean we need to fix things now because this PR is not ready yet, there are still warnings and errors in shell scripts.

[matthiasbeyer]

Can someone please tell me what to do about

• SC2153 in configure_bridge
• SC1090 in multiple locations
• SC1091 in multiple locations

[matthiasbeyer]

I am going to file issues for the things that are ignored in 5be1243, 4e52a28 and ea85c68 after this is merged.

[didier-wenzek]

• SC2153 in configure_bridge

The variable name is misleading but seems correct. This is supposed to run in an environment where some variable have been exported ($C8YDEVICE, but also $C8YUSERNAME ...). A way to fix the issue would be to add an explicit check at the beginning of the script to enforce these variables to be actually set.

• SC1090 in multiple locations
• SC1091 in multiple locations

These two seem more related to shellcheck configuration or script organisation.

[matthiasbeyer]

Squashed. :tada: