[NUCLEO_F302R8] Code formatting
[NUCLEO_F302R8] Add automatic HSE/HSI clock configuration + change spi default pins

The clock configuration is first tried with external 8MHz clock, if fail
then tried with 8MHz xtal and finally with HSI.
Merge pull request #232 from bcostm/master

[NUCLEO_F302R8] Improvements in clock configuration, spi default pins, code formatting