samr21: implemented 32 bit hwtimer
drivers/srf08: initial import of the updated ultrasonic driver
Merge pull request #2179 from PeterKietzmann/adapt_srf08_ultrasonic_driver

drivers/srf08: improved and adapted ultrasonic ranger driver to general i2c interface
Merge pull request #1998 from Troels51/samr21-hwtimer-fix

samr21: 32 hwtimer