hwd: require dtb to parse in order to use devicetree backend

hwd/src/backend/devicetree.rs

@@ -9,6 +9,7 @@ pub struct DeviceTreeBackend {
 impl Backend for DeviceTreeBackend {
     fn new() -> Result<Self, Box<dyn Error>> {
         let dtb = fs::read("/scheme/kernel.dtb")?;
+        let dt = fdt::Fdt::new(&dtb).map_err(|err| format!("failed to parse dtb: {}", err))?;
         Ok(Self { dtb })
     }