Please allow a configurable default UART bitrate

[harbaum]

Currently, the uart bitrate used for console or basic stdout via printf etc is hard coded to 2000000 bit/s in the board config like bsp/board/bl616dk/board.c:

https://github.com/bouffalolab/bouffalo_sdk/blob/302e017ea06b4c75963212f7144f8800c05901f1/bsp/board/bl616dk/board.c#L204

and

https://github.com/bouffalolab/bouffalo_sdk/blob/302e017ea06b4c75963212f7144f8800c05901f1/bsp/board/bl616dk/board.c#L214

Not all serial uart adapters work at this rate, and e.g. some cheap cp2102 based devices I have won't cope with that rate. Instead, 921600 bit/s works fine, so I need to patch the SDK to get this working.

It would be nice to have some place to adjust this setting on a per-project basis, so I can leave the SDK untouched when changing the bit rate.