On Teltonika RUTx devices, the init script fails due to failure to resolve interface name

[luckman212]

To get darkstat to initialize with any interface other than 'lan' (e.g. qmimux0), I had to change

packages/net/darkstat/files/darkstat.init

Lines 38 to 41 in d6ad088

	build_config_string() {
	local cfg="$1"
	config_get interface $cfg interface
	network_get_device ifname "$interface"

to

build_config_string() {
       local cfg="$1"                                  
       config_get interface $cfg interface 
       network_get_device ifname "$interface" || ifname="$interface"

# darkstat --version
darkstat 3.0.719 (using libpcap version 1.9.1 (with TPACKET_V3))

Teltonika RUTX11, firmware RUTX_R_00.07.10.2 based on OpenWrt 21.02