sys/vtimer: spin on short duration in vtimer_sleep

closes #571
board: Initial import of stm32f0discovery
cpu: Initial import of cortex-m0_common
cpu: Initial import of stm32f0
tests/examples: added stm32f0discovery to ignorelists
Merge pull request #1291 from haukepetersen/board_stm32f0discovery

Board stm32f0discovery
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
cpu: atmega_common: Initial import
cpu: atmega2560: Initial import
board: arduino-mega2560: Initial import
tests: blacklist board arduino-mega2560:

    - tests/bloom: value too large for field of 2 bytes
    - tests/<other>: missing headers (time.h or sys/types.h)
test: add download of avr toolchain to .travis.yml
tests: blacklist tests/test_coap and examples that fail due to missing header files