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
nrf51822: Suppress unused argument warnings
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
RIOT/Makefile.include: added variable `EXPECT_CONFLICT` to be used for indicating possibe conflicting features.
- For instance using `SPI0` and `DAC0` on the `stm32f4discovery` at the same time

The variable should be set in the board `Makefile.features`
[SQUASH ME or REMOVE ME] added small test