sim: Implement Default for a few types

Clippy suggests implementing Default when the `new` function for that
type just initializes the type to default values.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/sim/mcuboot-sys/src/area.rs b/sim/mcuboot-sys/src/area.rs
index dc05e39..82a7dfd 100644
--- a/sim/mcuboot-sys/src/area.rs
+++ b/sim/mcuboot-sys/src/area.rs
@@ -11,7 +11,7 @@
 use std::collections::HashMap;
 
 /// Structure to build up the boot area table.
-#[derive(Debug, Clone)]
+#[derive(Debug, Default, Clone)]
 pub struct AreaDesc {
     areas: Vec<Vec<FlashArea>>,
     whole: Vec<FlashArea>,