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.