west: runners: blackmagicprobe: update reset on connect command

Recently the blackmagicprobe command for "connect under reset" has been
updated from "connect_srst" to "connect_rst". Update the flag name in
the west blackmagicprobe binding, keep the old one as well for
compatibility with out of tree boards, and send both the old and new
commands to GDB so that it works with both the old and new firmware. GDB
is going to get a "command not supported by this target" for one of the
two and proceed with the other one.

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