pmbootstrap lint: use CUSTOM_VALID_OPTIONS
Let apkbuild-lint check options again, and pass the pmbootstrap specific options with the new CUSTOM_VALID_OPTIONS variable. Add a test case and adjust pmb.helpers.lint.check to return the output of apkbuild-lint, so we can properly test it.
- Related: https://gitlab.alpinelinux.org/Leo/atools/-/merge_requests/28 (merged)
- Fixes: pmaports#553 (closed)