WIP: added spark core board

TODO:
- [ ] test
- [ ] restore examples/hello-world
- [ ] extend blacklists
as we don't have a SPI-Flash driver, I would throw out these defines for now, or if you want to keep the reference to the flash in here, change to
```c
#define EXTFLASH_SPI     SPI_x
#define EXTFLASH_SPI_SPEED   SPI_SPEED_1MHZ   /* choose according to the flashes datasheet */
#define EXTFLASH_CS     GPIO_x
```
again, this is all you need...