How to specify an override for the default serial port used for flashing firmware via flash_firmware.sh?

## Issue template

- Hardware description: Ubuntu 20.04 (Agent), ESP32 (Client)
- RTOS: FreeRTOS
- Installation type: micro_ros_setup flash_firmware.sh
- Version or commit hash: galactic

#### Steps to reproduce the issue
Running *ros2 run micro_ros_setup flash_firmware.sh* defaults to the first available USB serial port. However, if I have a few MCUs connected -- say USB0, USB1, USB2 -- and I only want to flash the one connected to USB1, how would I go about doing this?

#### Expected behavior
flash_firmware.sh should offer an option (eg, --dev /dev/ttyUSB1) to flash to a particular port

#### Actual behavior
No such option exists. flash_firmware.sh flashes to the first available USB port

#### Additional information

Start by inspecting flash_firmware.sh and the ros2 run micro_ros_setup flash_firmware.sh entry point to see how the default USB serial port is selected. Add a way to pass a device such as /dev/ttyUSB1, then verify that flashing targets the requested port when multiple USB devices are connected.

Written by the indexing model from the issue text.