GitLab

- Only initialize CPU 0 and spin on other CPUs (based on https://github.com/rust-embedded/rust-raspberrypi-OS-tutorials/)
- Adjust physical start address of RAM to correct value for raspi3 (again based on the raspberry tutorial)
- Add feature to map write_char() to use a special address for usage with qemu. The aarch64 solution doesn't seem to work on qemu. Didn't look to closely though.
 The feature is enabled by default for make arch=aarch64 for now. A toggle is future work