If a board wants to customise the clocks it can define the following:
MBOOT_CLK_PLLM
MBOOT_CLK_PLLN
MBOOT_CLK_PLLP
MBOOT_CLK_PLLQ
MBOOT_CLK_PLLR (only needed on STM32H7)
MBOOT_FLASH_LATENCY
MBOOT_CLK_AHB_DIV
MBOOT_CLK_APB1_DIV
MBOOT_CLK_APB2_DIV
MBOOT_CLK_APB3_DIV (only needed on STM32H7)
MBOOT_CLK_APB4_DIV (only needed on STM32H7)
Signed-off-by: Damien George <damien@micropython.org>