Now all examples are including only the modules they really need. Also
each header file of the library is including only the necessary headers
making it possible to use these modules in parallel with other
implementations that may collide with the definitions in other modules.
For now, add the following basic SPI functions:
- spi_init_master()
- spi_write()
- spi_read()
This is incomplete and untested, yet.
Also, add some more SPI bit definition macros and comments.
