The configurability of the HCI Transport Layer for UART-based DFU in components/libraries/bootloader_dfu/hci_transport/hci_transport_config.h leaves something to be desired.
The following #defines should probably be wrapped in #ifndef statements, so that the #include "boards.h" can have an effect if BOARD_CUSTOM is defined and I define these settings there. Otherwise, I'd have to edit the HCI_SLIP_UART_BAUDRATE setting explicitly if I wanted a different one, and I would have to do that every time the SDK was updated.
#define HCI_SLIP_UART_BAUDRATE UART_BAUDRATE_BAUDRATE_Baud38400 /**< Defines the UART Baud rate. Default is 38400 baud. */
/** This section covers configurable parameters for the HCI Transport layer that are used for calculating correct value for the retransmission timer timeout. */
#define MAX_PACKET_SIZE_IN_BITS 8000u /**< Maximum size of a single application packet in bits. */
#define USED_BAUD_RATE 38400u /**< The used uart baudrate. */
