Add lint target to Makefile

[majst01]

What this PR does / why we need it: The code is actually not linted and contains a lot of code which needs to be fixed to pass basic linting rules.

With this PR introduce a make lint target which enables developers to quickly see what errors needs to be fixed.

Release note:

Introduce make lint target to show actual open issues in the codebase.

[gardener-robot]

@majst01 Thank you for your contribution.

[gardener-robot-ci-3]

Thank you @majst01 for your contribution. Before I can start building your PR, a member of the organization must set the required label(s) {'reviewed/ok-to-test'}. Once started, you can check the build status in the PR checks section below.