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 (closed)