Please add support for DT validation

***In GitLab by @broonie1 on Jun 1, 2022, 13:16 GMT+1:***

The kernel has support for validating the DT bindings via the dtbs_validate and dt_binding_check targets, it would be nice if tuxmake could be asked to do these checks (they are currently very computationally expensive so a bad candidate for doing by default).

These require that some tooling be available on the system (pip install git+https://redirect.github.com/devicetree-org/dt-schema.git@master) so would need some updates to the containers as well, or a separate container specifically for DT validation.

*Migrated from GitLab: https://gitlab.com/Linaro/tuxmake/-/issues/182*

Start by tracing how tuxmake exposes kernel make targets and how its build containers are defined. Check the dtbs_validate and dt_binding_check targets and the required devicetree schema tooling described in the issue. Done means users can request DT validation explicitly without making it run by default, with the required tooling available in the relevant container setup.

Written by the indexing model from the issue text.