ieee802154: introduce constant for default PAN id
cpu/samd21: add i2c periph implementation
doccheck: grep for all lines with a warning
core: `sched_switch()` switch if not on runqueue

Fixes #1935.

`sched_switch()` should not only switch if the other priority is higher,
but also if the current thread was moved from the runqueue.
Don't evaluate inISR() if not needed
tests: transformed tests/sha256 into an unittest
sys:net:routing:rpl made the number of routing entries configurable on compile time.
Merge pull request #2180 from OlegHahm/doccheck_fix_grep_sed

doccheck: grep for all lines with a warning
tools/licenses: add revised 1c-BSD pattern used in utlist
sys/include: Include utlist for simple list operation defines
Merge pull request #2174 from fnack/utlist

sys/include: Add utlist to sys includes
boards,drivers,examples: unset ENABLE_DEBUG
Merge pull request #2181 from phiros/tests_sha256_transform_into_unittest

tests: transformed tests/sha256 into a unittest
Merge pull request #2186 from LudwigOrtmann/disable-debug

boards,drivers,examples: unset ENABLE_DEBUG
Merge pull request #2169 from BytesGalore/rpl_configuarbale_max_routing_entries

sys:net:routing:rpl allow setting the number of routing entries on comile time
native: move rtc to periph

* adapt implementation to new interface
* remove now superfluous drivers directory
native/rtc: clean up documentation
native/rtc: improve implementation

* add error checking / warnings
* implement parts of alarm functionality
* clean up
sixlowpan: remove redundant assignment
sixlowpan: handle malloc error

The return value of malloc wasn't checked before, resulting in
possibly undefined behavior.