Reduce the flash latency to 2WS as that is sufficient for the 84MHz clock.
Can't use HSE clock on the Nucleo board as it's not populated.

The board as supplied does not provide any external clock to the F4.
It could supply an 8MHz clock from the STLINK part but, by default, it
isn't configured to do that.

So just use the HSI instead of the HSE - it won't be good for USB or fast
CAN bit rates but should be fine for bit banging, etc.
Now determines the required flash latency from the system clock frequency.

This assumes that the MCU supply voltage is at least 2.7V.
Rework configuration of STM32F4 HSI clock.

Now, to use the HSI clock, specify an internal clock frequency in the
"clocks" config section, like this...

clocks = { internal = 16000000, cpu = 84000000 }

Don't specify an external frequency as well as the internal frequency.
Merge pull request #47 from smartavionics/stm32f4-nucleo

Stm32f4 nucleo