vtimer: fix callback usage

There is no need to test the "handler" set in the vtimer struct, and
have some code executed then. We just can make the code to execute the
handler. To lengthy `if else if`, just a call.
core/msg: revert part of #1285

use m->sender_pid instead of sched_active_pid again
undoes the optimization intended by #1285
fixes uint/int warning
core/msg: set m->sender_pid before it is used

fixes `msg_send_to_self` condition
SQUASHME: do it right
sys/transceiver: fix t9r_register API compliance
sys/transceiver: implement transceiver_unregister
sys/transceiver: refactor un/register

remove race conditions
make less restrictive (allow double-un/registers)
make loops more readable
sys/transceiver: use size_t instead of u8 in loops
Merge pull request #1395 from LudwigOrtmann/transceiver_unregister

sys/transceiver: add transceiver_unregister and refactor transceiver_register
make: move exported make vars to one place
make: don't use export on example makefiles
Merge pull request #1109 from Kijewski/make-exports

make: no "unfriedly" `exports` in the application's Makefile
Merge pull request #1387 from LudwigOrtmann/msg_fixup

core/msg: clean up after #1285
Merge pull request #1034 from Kijewski/vtimer-callbacks

vtimer: fix callback usage
Initial commit of Arduino Mega2560
strip functions in atmega2560/periph and atmega_common/thread_arch.c down to prototypes

Add custom errno.h since avr-libc doesn't include one
rename board_init to main, main to o_main (inside hello-world/main.c). Fix
Makefile.include:

UART works within RIOT codebase, :)
Develop: fix invocation of avrdude
arduino-mega2560: cleanup, add interrupt driven uart communication
SQUASHME: return examples/hello-world/main.c to original state