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.
Initial import of nrf24l01p lowlevel functions
[SQUASM ME] removed tests/test_spi
[SQUASH ME] changed delay after function call
[SQUASH ME] removed netdev includes from drivers/makefile
[SQUASH ME] update some small things to changes caused by netdev in another PR
[SQUASH ME] added a small REAMDE to describe the usage of the test
[SQUASH ME] whitelist arduino-due for nrf24l01p lowlevel test
[SQUASH ME] style fixes
added msg_send function from rx-interrupt to thread
added documentation and changed some functions return types
[SQASH ME] removed spi transfers during ISR
[SQUASH ME] addressed comments by @LudwigOrtmann