custom board support

[ozanoner]

With this commit, custom boards are supported. To add a custom board:

1. Create a new header file with the name of "TinyMLx_custom_board.h". The best way to do that is to copy the "TinyMLx_board.h" file from the library and rename it, then update its content with your board definitions.
2. Provide "-DUSE_TinyMLx_custom_board" to the compiler and the path of the header file with -I flag.