simulavr: added exit register

Added an exit register definition for simul avr (SIME (SImulator Exit)).
Writing a value to this register exits the simulator and displays the
value written to the register as error code on the console. Can be used
in tests to exit with error codes.

Signed-off-by: Michel Megens <dev@michelmegens.net>
git: ignore: added an entry for vim swp files.

Vims .swp files can no longer accidently end up in the repository as
there is an entry for them in the .gitignore file now.

Signed-off-by: Michel Megens <dev@michelmegens.net>
usr: tests: added automated tests to usr/

Automated tests can be added to usr/ from now on. These tests can be
designed to run on real hardware or on simulators of real hardware. To
run all tests issue the following command in usr/

`make tests'

To run a specific test, cd into the usr/tests directory and run:

`make <testname>'

Signed-off-by: Michel Megens <dev@michelmegens.net>