Define the board's external crystal (HSE) frequency `xtalHz` in the
respective `board_*.go` files. Per-topology PLL tables (F1, F4, F7)
will compute the register dividers from it.
Moving this parameter to the board definition level cleanly isolates
board hardware characteristics from general MCU chip configurations,
eliminating the need for custom target build tags. Targets using
HSE-clocked STM32 chips must define `xtalHz` or fail to build.
Go 1.19 started reformatting code in a way that makes it more obvious
how it will be rendered on pkg.go.dev. It gets it almost right, but not
entirely. Therefore, I had to modify some of the comments so that they
are formatted correctly.
Konstantin Sharlaimov
Yurii Soldak
Ayke van Laethem
sago35
Elias Naur