drivers/cc2420: use DEBUG_ENABLED instead of DEBUG

compare https://github.com/RIOT-OS/RIOT/issues/2067
Merge pull request #2068 from LudwigOrtmann/debug-enabled

drivers/cc2420: use DEBUG_ENABLED instead of DEBUG
pkg/libcoap: call "$(MAKE)", not make
test/coap: update blacklist for features

Boards that don't have a transceiver are automatically filtered out already.
nrf51822: test arguments for read/write even in stubs
nrf51822: Fix indentation in lpm_arch.c
nrf51822: error was not checked b/c of wrong signedness
Merge pull request #2077 from LudwigOrtmann/libcoap-make

pkg/libcoap: call "$(MAKE)", not make; tests/coap: update blacklist
nrf51822: Suppress unused argument warnings
dist/licenses: add and use diff-filter
dist/licenses: add and use --error-exitcode
Merge pull request #2072 from LudwigOrtmann/license-difffilter

dist/licenses: add and use diff-filter
cpu: added `extern "C"` to headers
Merge pull request #2078 from Kijewski/hello-world-warnings

nrf51822: hello-world builds without warnings
Merge pull request #1803 from BytesGalore/extern_C_in_headers_cpus

cpu: added `extern "C"` to headers
native: move rtc to periph

* adapt implementation to new interface
* remove now superfluous drivers directory
native/rtc: clean up documentation
native/rtc: improve implementation

* add error checking / warnings
* implement parts of alarm functionality
* clean up
sys/shell: rewrite rtc command for periph/rtc
examples/default: use periph/rtc instead of rtc

The rtc module is deprecated and will be removed.