boards: nucleo_f103rb: Fix openocd configuration

With latest version of openocd delivered in Zephyr SDK 0.15.0,
a new configuration is required to flash and debug this board:
"reset_config connect_assert_srst" allows to flash the board
The new "init" function allows to run debug.

Fixes #50590 for this specific board

Note that other boards might be also impacted by this new version
of openocd but a revert of zephyrproject-rtos/openocd@98d9f11 allows
to get back to the previous status.
A new Zephyr SDK release (V0.15.1) will be available with a revert of
this commit. Unfortunately this has no impact on nucleo_f103rb. Hence
this change.


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