Introduce PUBLIC_BRIDGE_MTU variable to set br-ex MTU

This variable can be used to accommodate for underlying infrastructure
that does not provide full 1500-sized traffic, or maybe instead gives
access to Jumbo frames.

Change-Id: I38a80bac18673a30842a7b997d0669fed5aff976
Related-Bug: #1603268
diff --git a/functions b/functions
index 4f5e10a..5856578 100644
--- a/functions
+++ b/functions
@@ -637,6 +637,15 @@
     fi
 }
 
+
+# set_mtu - Set MTU on a device
+function set_mtu {
+    local dev=$1
+    local mtu=$2
+    sudo ip link set mtu $mtu dev $dev
+}
+
+
 # Restore xtrace
 $_XTRACE_FUNCTIONS