The board_hw_defs.c file should only contain static
const declarations that only describe the hardware.
All other non-const variables should be put into one
of the pios_board.c files.
The PIOS_MASTER_CLOCK has been removed in favor of
PIOS_SYSCLK (the master clock value,168000000 for revolution)
PIOS_PERIPHERAL_APB1_CLOCK and
PIOS_PERIPHERAL_APB2_CLOCK
Look at STM32F4xx_Revolution.h for the list of timer/peripheral and APB them belongs.
