add shellcheck action to catch script errors

[openoms]

will need to fix or ignore errors

Can be run locally with act https://github.com/nektos/act :

• install github cli https://cli.github.com/
• install act with github cli

gh extension install https://github.com/nektos/gh-act

• run this action

gh act -W '.github/workflows/test-shellcheck.yml'

[rootzoll]

Because this might force a lot of small changes - also in the sd card build area, lets target that change for v1.12.0

[openoms]

@rootzoll d52f15c and 68b478d are catching errors which affect the run of the scripts already.

Making these changes allows us to use shellcheck more rigorously so there are no new errors are introduced.

If there are false positives espiacally around using $@ vs "$@" we can ignore those lines explicitly.

[openoms]

Disabled checking the lines which are working correctly, but causing shellcheck errors. The rest are actual bugs.

[openoms]

@rootzoll should we include d52f15c separately?

[rootzoll]

@rootzoll should we include d52f15c separately?

@openoms cherry-picked into dev & v1.11