To use HSI instead of HSE define MICROPY_HW_CLK_USE_HSI as 1 in the board
configuration file. The default is to use HSE.
HSI has been made the default for the NUCLEO_F401RE board to serve as an
example, and because early revisions of this board need a hardware
modification to get HSE working.
This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.
