Some drivers or tests need to execute some code before Zephyr is
booted, dynamically register command line arguments, etc.
For this purpose, we generalize the NATIVE_EXIT_TASK to also
provide hooks to run a function at a given point during the startup
of native_posix.
Also, test/boards/native_posix/exit_tasks is generalized to cover
this new functionality.
Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>