The hardware timer used relative offsets that were never updated
before. This leads to two problems:
a) later timers will get pushed into the future by the amount of
   previous timers
b) if a short timer is set continuously, a longer timer will never be
   called

Example:
a)
Timer a with 500 ms is set, timer b with 600 ms is set.
timer a expires after 500 ms, timer b will be set to expire in 600 ms
which totals to 1100 ms.
b)
Timer a is set to 500 ms, timer b is set to 600 ms.
Timer a expires and is set again. Now timer a will expire in 500 ms
and timer b will be pushed further into the future. Repeating this
will lead to b never expiring.

reverts bug introduced by 2bfb0623

searching for the next pending timer was broken
this commit rewrites this functionality

make it behaver more like a regular timer

wrap some libc functions that do system calls (terminal output)
wrap read/write with syscall guard
define real_read/write (next dynamic linker find for read/write)
guard system calls in remaining code
introduce native_internhal.h
throw out some debug statements that break things
clean up includes a bit
declare board_init in native_internhal.h
add -ldl to LINKFLAGS for cpu/syscalls

not properly passing received data to transceiver atm

NOTE: this commit introduces a kernel API change for mutex_unlock

(including uart0 integration, rt-extension removal)

init hwtimer with interrupts enabled
more error checking for hwtimer
fix signal handler init in hwtimer
error checking for irq (not necessary yet afaik)

also fix and improve some of the existing