net_if: alternative approach for swapping the byte order
auto_init: always set source address mode
pyterm: enable TCP connection to remote host
pyterm: make mostly pep8 compliant
drivers/periph/rtt: added overflow callback
cpu/nrf51822: adjusted to RTT interface changes
cpu/stm32f1: adjusted to RTT interface changes
core: put more intelligence into `queue_msg()`

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()`.
pktbuf: optimize size overhead in packet buffer
make: remove downloader "axel"

Breaks for `make -B`. If the file already exists, then `axel` fails
because it does not find the status file for the download.
unittests: Add missing semicolons to macro calls
unittests: make test macros save for strict aliasing
c++: sys: add extern C to header files
samr21-xpro port with transceiver