twister: fix serial connection for flash_before

The --flash-before flag allows devices to be flashed before the serial
connection is established. However, the implementation was incomplete and
only worked if the port address already existed at the start of the run.
This is incompatible with devices that implement the USB in software
(eg: USB-CDC).

This commit fixes the implementation to delay setting up the connection
until after the device is flashed, and to retry the connection for two
seconds after flashing to give the device time to enumerate as a USB
device.

Signed-off-by: Mike Szczys <mike@golioth.io>