core: moved `#include`s outside the `extern "C"` guards
drivers: add `extern "C"` to header files
byteorder: silence warnings in clang
Merge pull request #1852 from authmillenon/byteorder-silence

byteorder: silence warnings in clang
Merge pull request #1805 from BytesGalore/extern_C_in_headers_core_unguard_includes

core: moved `#include`s outside the `extern "C"` guards
Merge pull request #1807 from BytesGalore/extern_C_in_headers_drivers

drivers: add `extern "C"` to header files