ieee802154: introduce constant for default PAN id
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
ccnl: adapt for 802154 devices
ccnl: fix argument passing in example app
ccn: add line breaks if limit was exceeded
ccnl: convert C++ comments into C style comments
examples:rpl_udp updated `README.md` on using the build parameters
Merge pull request #2192 from BytesGalore/rpl_addition_to_readme

examples:rpl_udp updated `README.md` on using the build parameters
doc: extend i2c description for pull-up
Merge pull request #2024 from OlegHahm/ccnl_802154

ccn-lite: fixes for working with IEEE 802.15.4 radios
Merge pull request #1958 from Kijewski/sched_switch-on-run-queue

core: `sched_switch()` switch if not on runqueue
Merge pull request #2199 from PeterKietzmann/adapt_i2c_doc

doc: extend i2c description for pull-up
driver/i2c: additional docu for pull resistors
Merge pull request #2200 from haukepetersen/fix_i2c_doc

driver/i2c: additional docu for pull resistors
native/rtc:  define ENABLE_DEBUG, move include
native/rtc: apply naming conventions