west: Fix --openocd-search argument

Running west flash with the --openocd-search arg will result in the
openocd command being run with a -s param added for every character in
the passed DIR.

Fix the argparser to append all openocd-search DIRs into a list, instead
of just passing a string to the subcommand.

Signed-off-by: Jeremy Bettis <jbettis@google.com>