Having the version macro in the config.h and consequently in the main
vehicle header means that whenever the version changes we need to
compiler the whole vehicle again. This would not be so bad if we weren't
also appending the git hash in the version. In this case, whenever we
commit to the repository we would need to recompile everything.
Move to a separate header that is include only by its users. Then
instead of compiling everything we will compile just a few files.
The switching between different AP_HAL was happening by giving different
definitions of AP_HAL_BOARD_DRIVER, and the programs would use it to
instantiate.
A program or library code would have to explicitly include (and depend)
on the concrete implementation of the HAL, even when using it only via
interface.
The proposed change move this dependency to be link time. There is a
AP_HAL::get_HAL() function that is used by the client code. Each
implementation of HAL provides its own definition of this function,
returning the appropriate concrete instance.
Since this replaces the job of AP_HAL_BOARD_DRIVER, the definition was
removed.
The static variables for PX4 and VRBRAIN were named differently to avoid
shadowing the extern symbol 'hal'.
Lucas De Marchi
Caio Marcelo de Oliveira Filho
Andrew Tridgell