If no explicit port given to upload_hex.sh, use SERIAL_PORT.
If no explicit port given to upload_hex.sh, use SERIAL_PORT.

This matches where the Makefile looks for the port when flashing.

mgiannikouris/vi-firmware,openxc/vi-firmware,openxc/vi-firmware,ene-ilies/vi-firmware,mgiannikouris/vi-firmware,mgiannikouris/vi-firmware,ene-ilies/vi-firmware,openxc/vi-firmware,mgiannikouris/vi-firmware,ene-ilies/vi-firmware,ene-ilies/vi-firmware,openxc/vi-firmware