WIP: added spark core board

TODO:
- [ ] test
- [ ] restore examples/hello-world
- [ ] extend blacklists
substitute this compete block with 
```c
#define BUTTON1_PIN     GPIO_x    /* which ever GPIO dev you map to the button pin */
#define BUTTON2_PIN  ...
...
```
Each button is mapped to exactly one RIOT `GPIO_x` device, and you read the button by
1. `gpio_init_in(GPIO_x);
2. `gpio_read(GPIO_x);