fff-babeld: add MTU option to babelpeers

Currently the MTU of a babelpeers is not configurable, nor set to a
default value. If present, the `mtu` option is now copied to the
interface section, otherwise it is set to a default value of 1500.

Warning for VLAN Interfaces:

For MTUs larger than 1500 to take effect, the parent interface also
needs adjusting. E.g. setting `eth0.100` to an MTU larger than the MTU
of `eth0` will not work.

At this point, manually add the following section to
`/etc/config/network`:

```
config interface
	option ifname 'eth0'
	option mtu '1570'
```

Beware that this will cause any child interface of `eth0` to have the
same large MTU.

Signed-off-by: Johannes Kimmel <fff@bareminimum.eu>

src/packages/fff/fff-babeld/Makefile

@@ -1,7 +1,7 @@
 include $(TOPDIR)/rules.mk
 
 PKG_NAME:=fff-babeld
-PKG_RELEASE:=7
+PKG_RELEASE:=8
 
 include $(INCLUDE_DIR)/package.mk
 

src/packages/fff/fff-babeld/files/etc/layer3.d/40-babel

@@ -35,6 +35,7 @@ configure() {
 		local prefixname="babelpeer_$name"
 		local vlan
 		local type
+		local mtu
 
 		# get iface
 		if vlan=$(uci -q get gateway.$name.vlan); then
@@ -52,10 +53,14 @@ configure() {
 		# get rxcost
 		rxcost=$(uci -q get gateway.$name.rxcost) || rxcost=96
 
+		# get mtu
+		mtu=$(uci -q get gateway.$name.mtu) || mtu=1500
+
 		# add interface
 		uci set network.$prefixname=interface
 		uci set network.$prefixname.proto=static
 		uci set network.$prefixname.ifname=$iface
+		uci set network.$prefixname.mtu=$mtu
 
 		# add iif-rules
 		babel_add_iifrules "$prefixname" || { echo "Could not add iif-rules for babelpeer $name"; exit 1; }