cpu/nrf51822: fixed gpio driver
boards: added LED macro for pca1000 & yunjia-nrf
core: Provide functions for different byte orders

Rationale: see #1586.
tests: Add unittests for byteorder.h
tests: fix bad copy+paste in tests-core.h
boards/iot-lab_M3: change to openocd's new ftdi interface

Openocd communication was done over a deprecated interface definition,
namely ft2232. Using the new ftdi interface speeds up flashing and
doesn't throw warnings anymore.
c++: core: add extern C in header files
Merge pull request #1779 from thomaseichinger/stm32f1-flashing

boards/iot-lab_M3: change to openocd's new ftdi interface
stm32f1: implement RTT driver
Merge pull request #1613 from thomaseichinger/stm32f1_rtt

stm32f1:iot-lab_M3: implement RTT driver
sixlowpan: fix ipv6_addr_init_prefix
Merge pull request #1781 from authmillenon/sixlowpan-fix-prefix-init

sixlowpan: fix ipv6_addr_init_prefix
Merge pull request #1783 from dangnhat/extern_C_in_headers_core

c++: core: add extern C in header files
Merge pull request #1699 from Kijewski/issue-1586

core: Provide functions for different byte orders
renamed the cpp header file to `*.hpp` to preven eventually a collision with #1789
Merge pull request #1792 from BytesGalore/rename_cpp_header_to_hpp

renamed the cpp header file from `*.h` to `*.hpp` to avoid false positive when require `extern "C"` in a C header.
make: make doc clean target ignore failures
make: add clean, distclean for RIOT

distclean
removes all build products, also depends on docclean

clean
removes all build products for the current board

also, silence clean target from Makefile.include and let it ignore errors
make: remove $(CLEANFILES), it's superfluous
udp: fix byte order in sockaddr6_t

udp_recvfrom wrote the sender port number in host byte order into the
provided sockaddr6_t. Because all send functions expect the port number
in network byte order this introduces a superfluous conversion step in
case one wants to reuse the address for replying.

closes #1406