feature/nosytare: empty kernel for baseline benchmark

This branch will add means to compile an "empty" kernel and automatically generate stubs for syscalls in order to benchmark an application without any Sytare overhead. This replaces the Sytare kernel when requested and doesn't require any changes to the application.

To allow benchmarking, a "finished" signal is sent after main() has terminated (run to completion of main is now our "app has finished" criterium) for both kernels. Furthermore another GPIO pin is used as input at boot to figure out if a reset is requested. This is needed to reset the board for the next T_on value.

The app "leds" has already been adapted for 'run-to-completion".

How to test

$ mkdir src/build
$ cd src/build
$ cmake .. -DNOSYTARE=True
$ make app_leds