The STM32F469 can use the same initialization as the existing STM32F407
with a few frequency tweaks. This change splits the generic
initialization code into a separate runtime_stm32f4.go file, leaving
only the 407 board specific constants in the existing
runtime_stm32f407.go file.
Note that runtime_stm32f405.go initialization seems semantically similar
to the 407, but I don't have enough confidence in merging 405 with 407
in this change.
