PLL3-Q is more reliable than PLL1-Q for the USB clock source when entering
mboot from various reset states (eg power on vs MCU reset). (It was found
that if the main application used PLL3-Q then sometimes the USB clock
source would stay stuck on PLL3-Q and not switch to PLL1-Q after a reset.)
Other related changes:
- SystemCoreClockUpdate() should be called on H7 because the calculation
can be involved in some cases.
- __set_PRIMASK(0) should be called because on H7 the built-in ST DFU
bootloader exits with IRQs disabled.
Signed-off-by: Damien George <damien@micropython.org>
c8c229b96c