boards/stm32f3: fix openocd config to flash from invalid state

When flashing multiple times the flasher sometimes gets stuck which
prevents flashing after.
It may be from a specific firmware or operation but do not have one yet.
Connect under reset fix flashing from this state.
-c 'reset halt' is needed to support debug with the new configuration.

It was found after the `stm32f3discovery` get stuck in a non-flashable mode
after some firmwares.

This was adapted from `stm3f103c8` configuration and commit.

Co-authored-by: MrKevinWeiss <weiss.kevin604@gmail.com>

boards/common/stm32/dist/stm32f3.cfg

@@ -1,3 +1,5 @@
 source [find target/stm32f3x.cfg]
-reset_config srst_only
+# use hardware reset, connect under reset
+# connect_assert_srst needed if low power mode application running (WFI...)
+reset_config srst_only connect_assert_srst
 $_TARGETNAME configure -rtos auto

boards/stm32f3discovery/Makefile.include

@@ -12,5 +12,9 @@ include $(RIOTMAKE)/tools/serial.inc.mk
 export DEBUG_ADAPTER ?= stlink
 export STLINK_VERSION ?= 2
 
+# call a 'reset halt' command before starting the debugger
+# it is required as `connect_assert_srst` is set
+export OPENOCD_DBG_START_CMD = -c 'reset halt'
+
 # this board uses openocd
 include $(RIOTMAKE)/tools/openocd.inc.mk