sim: Move `areadesc` into `Images` struct

Move this struct into the images struct as well to avoid passing it
around with every call.

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 5a1ef6c..c4f437e 100644
--- a/sim/mcuboot-sys/src/area.rs
+++ b/sim/mcuboot-sys/src/area.rs
@@ -4,7 +4,7 @@
 use std::ptr;
 
 /// Structure to build up the boot area table.
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 pub struct AreaDesc {
     areas: Vec<Vec<FlashArea>>,
     whole: Vec<FlashArea>,