boards: Fix openocd debug config on stm32h7 based boards

On these 3 stm32h7 based boards, 'connect_assert_srst' should
be used in order to be able to program after board unplug/plug.

Problem is that 'connect_assert_srst' prevents gdb-attach procedure
to complete which now requires 'reset halt' to be performed before
hand.
Fix this by forcing 'reset halt' by introducing a new init routine.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>