bsp/nucleo-f103rb: Use HSE instead of HSI
bsp/nucleo-f103rb: Use HSE instead of HSI

Nucleo board have 8MHz clock generated by on-board
st-link debugger.
This clock has higher accuracy then internal HSI.

Changing from HSI to HSE (both 8MHz) results in:
- System clock increased from 64MHz to 72MHz (maximum)
- Peripherals can operate on higher baud rates.
- Greater stability of system clock that may impact USB
  transfers.

mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core