sys/transceiver: cc2420 handles 802.15.4 packets
shell/transceiver: properly initialize 802.15.4 packet
added missing `extern "C"` guards
Merge pull request #1856 from BytesGalore/extern_C_in_headers_drivers_add_new_headers

drivers: added missing `extern "C"` guards in new files
cpu: use typed function pointer for thread_arch_init
Merge pull request #1866 from OlegHahm/core_fix_thread_init_prototype

cpu: use typed function pointer for thread_arch_init
cppcheck: don't needlessly assign sched_active_pid
Add copyright information to lpc1768/include/cpu.h
Merge pull request #1830 from thomaseichinger/transceiver-fixes

sys/transceiver: fixes for the use of `ieee802154_packet_t`
vtimer: improve documentation of vtimer_set_msg()
Merge pull request #1869 from authmillenon/vtimer-doc-improve

vtimer: improve documentation of vtimer_set_msg()
core: introduce thread_yield_higher(), yield less

Fixes #1708.

Currently involuntary preemption causes the current thread not only to
yield for a higher prioritized thread, but all other threads of its own
priority class, too.

This PR adds the function `thread_yield_higher()`, which will yield the
current thread in favor of higher prioritized functions, but not for
threads of its own priority class.

Boards now need to implement `thread_yield_higher()` instead of
`thread_yield()`, but `COREIF_NG` boards are not affected in any way.

`thread_yield()` retains its old meaning: yield for every thread that
has the same or a higher priority.

This PR does not touch the occurrences of `thread_yield()` in the periph
drivers, because the author of this PR did not look into the logic of
the various driver implementations.
Merge pull request #1836 from Kijewski/yield-less

core: introduce sched_yield(), yield less