[qmp-system] Add qmp_get_openwrt_default_network function

The qmp_get_openwrt_default_network() function fetches the
/etc/board.json file to obtain the default interfaces for the
lan and wan network roles.
Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>

packages/qmp-system/files/etc/qmp/qmp_functions.sh

@@ -43,6 +43,7 @@ qmp_check_device() {
 	ip link show $1 1> /dev/null 2>/dev/null
 	return $?
 }
+
 # Function qmp_set_vlan()
 #
 # This function creates a VLAN interface on top of an interface in order to
@@ -284,6 +285,16 @@ qmp_configure_smart_network() {
 	qmp_uci_set interfaces.ignore_devices "$ignore_devs"
 }
 
+qmp_get_openwrt_default_network() {
+	local role=$1
+	local board_file="/etc/board.json"
+	local flen=$(wc -l $board_file | cut -d " " -f 1)
+
+	[ "$role" != "lan" ] && [ "$role" != "wan" ] && return
+
+	grep -A${flen} "network" $board_file | grep -A${flen} $role | grep -m 1 -B${flen} "}" | grep -m 1 "ifname" | cut -d ":" -f2 | sed -e 's/^[ \t]*//' | cut -d '"' -f 2
+}
+
 qmp_attach_device_to_interface() {
 	local device=$1
 	local interface=$2