tests: let license checker check headers, too
changed parameters form `kernel_pid_t` to `int`
pthread: pthread_barrier should call yield
tests: clean up test_pthread_barrier Makefile
make: don't call mkdir -p a lot
make: use $@ and $<
make: call ranlib
make: don't invoke wildcard repeatedly
make: easier to read object targets
sys: remove lib/hashtable.c

Quoting myself:

> The code of sys/lib/hashtable.c is very badly adapted for embedded
> systems. The used primes are huge, and the limit is entirely
> unrealistic. Why do we ship this file?

In #1564 we came to the conclusion that we don't need this code.

Closes #1564.
core: add `thread_get()`

Remove PID check duplication in `thread_getstatus()` and
`thread_getname()`.
Merge pull request #1562 from Kijewski/the-first-two-commits-of-the-branch-issue-198

posix: minor pthread_barrier clean up
pyterm: fix exit behavior if twisted is not available
Merge pull request #1582 from N8Fear/fix_pyterm_twisted

pyterm: fix exit behavior if twisted is not available
Merge pull request #1580 from Kijewski/thread_get

core: add `thread_get()`
core: introduce KERNEL_PID_FIRST and KERNEL_PID_LAST
core: let valid PIDs start with 1
all over the place: use sched_active_pid

In many places we needlessly use `sched_active_thread->pid` whilst we
already have `sched_active_pid` with the same value, and one less
indirection.

`thread_getpid()` is made `static inline` so that there is no penalty in
using this function over accessing `sched_active_pid` directly.
core: SCHED_PRIO_LEVELS==16 for every board

Closes #1399.

> Using a different value for SCHED_PRIO_LEVELS for 16 and 32 bit
platforms hurts portability, one thing that we heavily advertise about
RIOT. if you want to write a portable application, then you have to
assume the lower value.

This PR defaults `SCHED_PRIO_LEVELS` to 16 for every board.
make: added CPU_MODEL & TERMFLAGS to Makefile.vars

- added CPU_MODEL variable
- added TERMFLAGS variable
- fixed a little style (dots on the end of comments)