core: put more intelligence into `queue_msg()`

drivers: added enhanced RTT driver interface

Fixes #1942.

There were two instances were it was not checked the target thread has a
message queue before queuing the message.

This PR centralizes the check into `queue_msg()`.

pyterm: enable TCP connection to remote host

auto_init: always set source address mode

Netif fix byteorder

tests/periph_spi: print rx-buffer on slave

tests/periph_adc: added test for adc driver

core: msg_receive should yield

cpu/stm32f0: added missing features to Makefile

If a thread sends blocking, but the target thread is not currently in
receive mode, the sender gets queued. If it has a higher priority it
should run again as soon as the target goes into receiving mode.

cpu/stm32f1: fix wrong bit operation in SPI init

shell: provide shell commands to poll lsm303dlhc sensor

shell: sensors: exit on failure

Immediately leave the read command if sensor is not initialized.

sys/shell: added commands for iot-lab_M3 sensors

drivers: added support for L3G4200D gyroscope

Ieee802154: use enums for frame type

iot-lab_M3: set baud rate to 500 kbaud

The IoT-Lab testbed requires a baud rate of 500 kbaud to use UART.
Hence, this commit sets the default baud rate to this value and adapt
the `make term` target accordingly.

drivers/at86rf231: improve deployment by default example