changed parameters form `kernel_pid_t` to `int`
dist/tools: add cppcheck script

enables easy linting of the whole tree or just one branches changes against
another
Fix thread_yield by avoiding the (too) early re-enablement of IRQ,
that is: before the newly selected thread's context is totally restored
Prevent the compiler to add an unwanted prologue in thread_yield()
Fix the race condition when an interruption occured at the beginning
of thread_yield(), i.e.: just after SR(R2) was pushed but before
the rest of the suspended thread's context was pushed!
SQUASHME: file(line) -> file:line

Addresses https://github.com/RIOT-OS/RIOT/pull/1604#discussion_r16936195
SQUASHME: add documentation
cpu: atmega: doxygen: blacklist upstream headers
Merge pull request #1642 from N8Fear/avr-libc-fix-doxygen

cpu: atmega: doxygen: blacklist upstream headers
SQUASHME: undo accidental commit of timex.h
SQUASHME: tool -> tools path typo
Merge pull request #1604 from LudwigOrtmann/cppcheck-script

dist/tools: add cppcheck script
vtimer: no needless type casting

Compare https://github.com/RIOT-OS/RIOT/pull/1639#issuecomment-54713048
and following discussion.
Fixed comments
tests: hwtimer shorten wait period

A sub-two-second period is needed for 16 bit timers.
tests/hwtimer: add self as author
Merge pull request #1643 from LudwigOrtmann/tests-hwtimer-shorten

tests: hwtimer shorten wait period
Merge pull request #1644 from Kijewski/vtimer-less-casting

vtimer: no needless type casting
Merge pull request #1639 from authmillenon/simplify_queue_inheritance

core: priority_queue: simplify inheritance
sixlowpan: ignore incoming packets if unconfigured

If there's no IPv6 address configured so far to any interface, 6lowpan
should not try to  handle incoming packets. This can easily lead to
looping packets.