When `OPENOCD_DEBUG_ADAPTER` was set to `stlink`, no `openocd.cfg`
was needed for most STM32 boards. (But one could still be provided if
special handling was needed.) However, for any other debug adapter this
was not the case.
This moves the handling for when no custom `openocd.cfg` is provided from
the `stlink.inc.mk` to `openocd.inc.mk`, so that most boards now work
out of the box with other debuggers.
Co-authored-by: crasbe <crasbe@gmail.com>
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