# save as:
#   /etc/systemd/system/serial-getty@ttyUSB0.service.d/custom.conf
# then reload config with
#   sudo systemctl daemon-reload
# and (re)start serial-getty@ttyUSB0.service

[Service]

# clear the list ExecStart list
ExecStart=

# tweaked version of the line in /lib/systemd/system/serial-getty@.service:
ExecStart=-/sbin/agetty -o '-p -- \\u' 115200 - $TERM