udev: network-hotplug-bridges: Silence a warning when interfaces have gone away

It can happen that udev has an event for an interface in the queue that
has already gone away - or even just being renamed.

Then reading the MAC address fails. Because the shell expands the
"$(<...)" statement before running the whole line, the read check is
useless.

Because the code would get too complicated otherwise, I decided to use
cat. Not cool, but this does the job.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>
Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>

config/udev/network-hotplug-bridges

@@ -42,7 +42,7 @@ detect_zone() {
 		local slave
 		for slave in $(get_value "${zone}_SLAVES"); do
 			# Compare if the mac address matches or if the name matches
-			if [ -r "/sys/class/net/${INTERFACE}/address" -a "$(</sys/class/net/${INTERFACE}/address)" = "${slave}" ] || [ "${INTERFACE}" = "${slave}" ]; then
+			if [ "$(cat /sys/class/net/${INTERFACE}/address 2>/dev/null)" = "${slave}" ] || [ "${INTERFACE}" = "${slave}" ]; then
 				echo "${zone}"
 				return 0
 			fi