Francisco J. Manno f938e70c69 stm32: Add compile-time option to use HSI as clock source.
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.
2019-03-05 15:49:08 +11:00
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