REPL working on UART6 with STMHAL

I put SystemClock_Config in system_stm32f4xx.c since it seemed to belong there.

HAL_Init already does the NVIC_ priority initialization

The uart stuff is hacked and only works on UART6 right now. The hal now has a whole structure associated with the UART and allows for tx and rx buffer to be used. So I think we need to fix the python bindings to allocate a uart object (and perhaps keep track of them so that if you ask for the same uart object multiple times we give back the already existing one).

And BOARD is currently set to STM32F4DISC. I'll flip it back to PYBOARD4 on the next PR unless you beat me to it.

I put #if 0 around anything I commented out (so we can search for #if 0). If I really wanted an #if 1 then I did #if 0 followed by a #else