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