Do not set hint.isp.X.role on upgrade if it is already set

(cherry picked from commit b2246a81da)

build/profiles/freenas/packages/base-os/config

@@ -31,7 +31,11 @@ post-install =
 
 	if ! dmidecode -s system-product-name | egrep "^(FREE|TRUE)NAS" > /dev/null; then
 		for i in `seq 0 3`; do
-			echo "hint.isp.$i.role=2" >> /boot/loader.conf.local
+			file=/boot/loader.conf.local
+			name="hint.isp.$i.role"
+			if ! cat "$file" | grep -q "$name"; then
+				echo "$name=2" >> $file
+			fi
 		done
 	fi