ieee802154: introduce constant for default PAN id
cpu/samd21: add i2c periph implementation
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
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
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
added a core implementation of FIB tables using generic addresses in an outer table structure
The FIB and the generic addresses operating event driven (no threads for either)
Access to either data is mutually excluded

- the basic functions are implemented for operating FIB tables (add/remove/update entry)
- Reactive Routing Protocols are notified when no destination is available for a next hop request
- generic addresses are implemented using a use_count to trigger if an address can be reused/overwritten
  once allocated memory will remain allocated until calling deintialisation for `generic_address`, pruning all entries
  or when an unsed entry is overwritten with a new entry of distinct size
- FIB tables are statically represented by an array of entries pointing to generic_address entries

TODO:
- test the implementation with the real network layer (tested with RPL on IoT-lab_M3)
- using the type field to recognize/apply compression
  - use compression, e.g. LOWPAN_IPHC

NOTE:
this is a prototype implementation it can be considered as a proof of concept in this state
It follows strictly KISS. So, no fancy algorythms nor code hocus-pocus is applied.