boards: add support for SODAQ Autonomo

The following features were tested (briefly):
* UART (the first)
* I2C using a BMP180 on a SODAQ TPH board
* SPI0, connected to the on-board serial data flash (only DevID was read
using the periph_spi test program)
* xtimer is working (it's the same code as in samr21)

boards/sodaq-autonomo: Disable the "big" unittests because it does not fit