scripts: runners: add connect_srst support for blackmagicprobe

Black Magic Probe supports a "monitor connect_srst" command [1] to
configure whether to assert reset when connecting to the target. This is
useful to reprogram a target that may be idling in a low power state,
with an otherwise unresponsive debug core.

Adding a --connect-srst runner option for enabling this for "flash" and
"debug" operations to ensure that programming works in that case, but
also explicitly disabling it for "attach", to allow attaching to a
running target without changing its state.

Not turning this on by default since apparently some MCUs disable the
SWD interface while in reset.

[1] https://github.com/blacksphere/blackmagic/wiki/Useful-GDB-commands



Signed-off-by: Fabio Baltieri <fabio.baltieri@gmail.com>