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:

bouffalo_sdk/bsp/board/bl616dk/board.c

Line 204 in 302e017

bflb_wo_uart_init(wo, 2000000, GPIO_PIN_21);

and

bouffalo_sdk/bsp/board/bl616dk/board.c

Line 214 in 302e017

cfg.baudrate = 2000000;

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.