Align custom kernel parameters with extensibility documentation

[SteffenBoThomsen]

Problem

Currently the extensibility documentation lists that customizations to kernel parameters can be done by

custom_parameters:
  common:
    - { ... }

This is not the case as the code expects them grouped under the distribtuion_id.

Solution

This PR ensures that both common and distribution_id can be utilized to set kernel parameters.
If both common and distribution_id sets the same property, the distribution_id will take precedence.
See the following example:

Which results in parameters being set like the following on the os level

[devanshjainms]

Thanks for the contribution. @SteffenBoThomsen