ieee802154: introduce constant for default PAN id
doc: fix doxygen group for cc2538 memory map
cpu/samd21: add i2c periph implementation
doc: add documentation for packet format in mc1322x
doc: order the main modules in a more sensible way
doc: remove superfluous and erroneous @file params
doc: don't use defgroup and addtogroup
doc: cleanup for radio_driver
doc: put rpl_storing header to right doxygen group
doc: add or move module description where missing
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
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
make: migrate OS variable for COLOR_ECHO too
Merge pull request #2196 from thomaseichinger/fix_color_echo

make: migrate OS variable for COLOR_ECHO too
Merge pull request #2016 from bapclenet/master

cpu/samd21: I2C driver implementation (Master mode)
ccnl: adapt for 802154 devices
ccnl: fix argument passing in example app