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.