[dox] fix some doxygen file blocks for fixedwing firmware
[actuators] throw informative error if still attempting to use MIN|MAX_MOTOR with new motor_mixing
[conf] remove duplicates after setting all flags and sources
Revert "[conf] remove duplicates after setting all flags and sources"
Doesn't work as intended (yet).
The sorting e.g. splits up `pkg-config glib-2.0 --cflags` into
--cflags` glib-2.0 `pkg-config
so it doesn't work anymore...
One way could be to make sure to resolve these first before sorting.

This reverts commit 70168207abc7f5c2b6a0fc766e03e8bf2b633009.
[conf] use a common spi.makefile to include the sources and general SPI flags
[build_system] removing duplicates for all airborne targets should work now.
pkg-config and the like need to be expanded via $(shell instead of using backquotes.
No spaces between -I and dir for generated module includes"
[dox] extract only documented files, strip code comments again...
[dox] okok... extract all again, but we should really just document the ones we want and only extract those
removed stray endif
[state_interface] fix float hmsl, thx Freek
[state_interface] fix validity test functions to not modify state.pos_status