This centralized namespace header encourages centralizing things on
umbrella headers that are a pain to maintain. Force each part of
AP_HAL_Linux to include what is used.
While at it, do some whitespace cleanups and minor changes to adhere to
coding style.
Instead of just doing a static cast to the desired class, use a method
named "from". Pros:
- When we have data shared on the parent class, the code is cleaner in
child class when it needs to access this data. Almost all the data
we use in AP_HAL benefits from this
- There's a minimal type checking because now we are using a method
that can only receive the type of the parent class