Force ping to use a specific address.

This helps when there are multiple networks available on the device running
tuya-convert, as it will force the pings to go via the interface that the
device will convert to, not the default interface.

start_flash.sh

@@ -75,7 +75,7 @@ while true; do
 	# The intermediate firmware will request 10.42.42.42
 	# Do NOT change this address!!!
 	# It will NOT make it install and will break this script
-	while ! ping -c 1 -W 1 -n 10.42.42.42 &> /dev/null; do
+	while ! ping -c 1 -W 1 -n 10.42.42.42 -S 10.42.42.1 &> /dev/null; do
 		printf .
 		if (( --i == 0 )); then
 			echo