* Implemented boost mode dance (RM0440 p234-245, 6.5.1)
* Enabled boost mode in usb_serial example, tested on hardware
* Removed hard requirement of a valid 48MHz source (HSI48 is checked if
requested, PLL passed through as-is and assumed to be valid)
* Used calc_pclk to calculate APB frequencies
* Refactored 48MHz configuration code to remove unnecessary let and block
* Renamed ahb_freq to hclk for clarity and consistency
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