drivers - spi: extended low-level spi driver interface
cpu - multiple: adapt spi drivers to interface extension
Merge pull request #1770 from fnack/spi_extension

driver - periph: Extend SPI low-level driver interface
c++: tests: add `extern "C"` in header files, i.e. only present in `./unittests`
Merge pull request #1788 from BytesGalore/extern_C_in_headers_tests

c++: tests/unittests: add extern C in header files