Check bitbake recipes with linter

The job checks bitbake recipes added/updated in MR using the
oelint-adv tool (https://github.com/priv-kweihmann/oelint-adv).

.oelint.json is a rulefile, created with the command
"oelint-adv --print-rulefile". It can be used during development.
It contains default rules for now, but will probably be customized later.

To get the list of changed files in MR, we adjusted the GIT_DEPTH variable.
The new value is 50, which should be fine for getting MR changes,
and still small enough to keep checkout fast for large projects.