Introducing shellcheck in gitlab CI

archiso/mkarchiso:
Quoting all variables.
Changing pkg_list to be an array instead of a string for easier
handling. Using read to properly populate pkg_list from OPTARG with
stripped whitespaces.
Not exporting iso_label anymore as there seems to be no reason to do so.
Introducing line breaks.

.editorconfig:
Setting max_line_length to 120.
Adding a section for YAML files (e.g. .gitlab-ci.yml).

configs/releng/build.sh
Quting nearly all variables.
Introducing line breaks.

configs/baseline/build.sh:
Quoting all variables.
Introducing line breaks.

.gitlab-ci.yml:
Adding gitlab CI for shelleck linting of the config build scripts, mkarchiso and startup scripts in releng.

Closes #19