make: don't call mkdir -p a lot
make: Make BOARDS a global macro for buildtests
[SQUASH ME] make: info-boards-supported is just
make: Add supported boards to info-build target
make: use $@ and $<
make: call ranlib
make: don't invoke wildcard repeatedly
make: easier to read object targets
[SQUASH ME] make: remove debugging lines
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.
[SQUASH ME] make: Move buildtest related PHONY targets to Makefile.buildtests
Merge pull request #1557 from authmillenon/make-list-show

make: Add target to show if boards that are supported by apps
native: remove redeclarations of sched_active_thread
Merge pull request #1574 from Kijewski/native-sched_active_thread

native: remove redeclarations of sched_active_thread
core: add `thread_get()`

Remove PID check duplication in `thread_getstatus()` and
`thread_getname()`.
board/stm32f3discover: fixed LED macros
cpu/stm32f3: let LD3 blink on hardFault
Merge pull request #1576 from haukepetersen/fix_stm32f0_leds

cpu/board: fixes to stm32f0(discovery)
drivers: improved the low-level GPIO driver IF

- removed UNDEFINED value from device enum
- changed most functions to void return value
- add parameter for interrupt callback
cpus: adjusted uart implementations for changed IF

- adjusted stm32f0
- adjusted stm32f3
- adjusted stm32f4
- adjusted sam3x8e
- adjusted nrf51822