sys/vtimer: spin on short duration in vtimer_sleep

closes #571
shell: prepare a rudimentary IEEE 802.15.4 packet

The transceiver module expects an `ieee802154_packet_t` instead of a
`radio_packet_t` if the device supports the IEEE 802.15.4 packet format.
This commit fixes the corresponding transceiver shell command for
`txtsnd` to set destination address (short address mode), payload, and
length accordingly.
Merge pull request #1434 from OlegHahm/fix_txtsnd_ieee802.15.4

shell: prepare a rudimentary IEEE 802.15.4 packet
board: Initial import of stm32f4discovery
cpu: Initial import of stm32f4
cpu: Initial import of cortex-m4_common
test/examples: blacklisted stm32f4discovery
Merge pull request #1191 from haukepetersen/board_stm32f4discovery

board: Initial import of stm32f4discovery board
Merge pull request #1407 from LudwigOrtmann/issue-571

sys/vtimer: spin on short duration in vtimer_sleep
vtimer: fix crash on spurious hwtimer callback

It seems that there are corner cases where a vtimer was removed, but
still there comes a hwtimer callback. This is a bug somewhere in the
vtimer or hwtimer. If there still was a vtimer set, then the next one
gets called before its time. If there was no other vtimer scheduled,
then `timer->action(timer)` crashes.

This PR simply fixes the crash, but does not attempt to find the more
fundamental bug.
Make filenames in periph documentation unique

This makes the filenames in the documentation of low-lever peripheral
drivers unique, so doxygen stops complaining about duplicates.
ccnl: use a second helper thread to handle e.g. timeouts
ccnl: use vtimer instead of rtc
ccnl: make theRelay use dynamic memory
ccnl: make relay start on RIOT startup
ccnl: use stacksize main for all threads
ccnl: prevent race condition on teardown
ccnl: adapt HOWTO to new features
ccnl: change timeouts to useful values
ccnl: normalize time before using it