PX4 platform init

[dagar]

Slowly establishing a good definition of the "PX4 platform" and now gradually pulling all the common elements together so that a system intended to run PX4 modules has all the prerequisites in place.

Reference - #11232 (comment)
#11261

[dagar]

For the most part the layers start to become evident when you look at the dependencies as a graph.

• modules and drivers should only interact through uORB and parameters
• code that can be shared between multiple modules and drivers is a library
• code that is shared between multiple modules, drivers, AND libraries forms the "PX4 Platform" layer

Other pieces to consider next:

• move uORB to platform layer, autostart in px4_platform_init, split out command line for status, top, etc
• move parameters to platform layer
• move perf to platform layer
• move systemlib to libs

[davids5]

Perfect!