sim: Don't take address of movable struct

This struct was having addresses taken of fields within it, and then being
returned.  It is platform-specific whether this causes a move.  It seems to be
working on x86_64, but causes a segfault on aarch64.  Box the struct so that it
isn't moved after being initialized.

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 95eea41..1ebb405 100644
--- a/sim/mcuboot-sys/src/area.rs
+++ b/sim/mcuboot-sys/src/area.rs
@@ -9,6 +9,7 @@
 use simflash::{Flash, SimFlash, Sector};
 use std::ptr;
 use std::collections::HashMap;
+use std::borrow::BorrowMut;
 
 /// Structure to build up the boot area table.
 #[derive(Debug, Default, Clone)]
@@ -124,8 +125,9 @@
         None
     }
 
-    pub fn get_c(&self) -> CAreaDesc {
-        let mut areas: CAreaDesc = Default::default();
+    pub fn get_c(&self) -> Box<CAreaDesc> {
+        let mut areas_box: Box<CAreaDesc> = Box::new(Default::default());
+        let areas: &mut CAreaDesc = areas_box.borrow_mut();
 
         assert_eq!(self.areas.len(), self.whole.len());
 
@@ -140,7 +142,7 @@
 
         areas.num_slots = self.areas.len() as u32;
 
-        areas
+        areas_box
     }
 
     /// Return an iterator over all `FlashArea`s present.