boards: arm: stm32: Fix openocd warning

interface/stlink-v2-1.cfg and interface/stlink-v2.cfg are wrappers
around interface/stlink.cfg, their inclusion trigger warning which
this change addresses. Besides the warning there is nothing there
except sourcing iterface/stlink.cfg directly.

Signed-off-by: Vasili Slapik <vslapik@gmail.com>