cpu/nrf51822: fixed gpio driver
boards: added LED macro for pca1000 & yunjia-nrf
make: del no-delete-null-pointer-checks for clang++

This PR enables the use of clang++ to compile C++ projects by adding
`CXXUWFLAGS += -fno-delete-null-pointer-checks`, if `${CC}` understands
`-fno-delete-null-pointer-checks` but `${CXX}` does not.
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
examples/default: set src addr correctly
examples/default: fix handling of 802.15.4 devices
shell/transceiver: fix byte selection for 802.15.4 devices in output
sys/transceiver: cleanup and fix for cc2420 and at86rf231

cc2420: the payload pointer was set twice
at86rf231: payload_len was not copied from the radio driver buffer
make: fix the order of Makefile.dep for timex

The order of modules in Makefile.dep matters.
make: fix dependencies

uart0 depends on POSIX, not the other way around.
posix: configure stdio only to uart0 if enabled
Merge pull request #1674 from thomaseichinger/default_fix

examples/default:sys/transceiver: fix for 802.15.4 radio devices
boards/iot-lab_M3: fix PORT
Merge pull request #1754 from OlegHahm/timex_makefile_order

make: fix the order of Makefile.dep for timex
Merge pull request #1773 from thomaseichinger/fix_iot-lab_term

boards/iot-lab_M3: fix PORT for TERM
netdev: Provide maximum value of netdev_opt_t for further definitions
Merge pull request #1774 from authmillenon/netdev-opt-max

netdev: Provide maximum value of netdev_opt_t for further definitions
netdev.tests: Add fixes for environment variables

+ minor include fix