CRC generation: Fixed linkage for C++ compilation (unit tests).

Added required extern "C" declarations.
Unit tests: modified the name of the generated CRC table.

The name was in conflict with the table generated for the actual application
due to the use of VPATH in the Makefile.
Code refactoring: modified the way the code is configured for the board.

Renamed and moved the 'settings.h' file to 'config/board_config.h'.
Moved the configuration function ('settings()', now called
'board_config()') to a C file ('board_config.c') to avoid duplicates
at link time.

Removed warnings due to deprecated register names (RBx) by using
the 'PORTBbits.RBx' syntax.