sim: Make the test builder optional

Some of the simulated devices aren't large enough to support the 5 slots
needed to test a multi-image configuration.  To allow this to work, make
the return from the `ImagesBuilder` return an option, so that it will be
able to indicate (with `None`) that this configuration isn't possible to
test, and that the test should be skipped.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/sim/src/lib.rs b/sim/src/lib.rs
index 58f5c59..43a1e5a 100644
--- a/sim/src/lib.rs
+++ b/sim/src/lib.rs
@@ -161,7 +161,13 @@
     pub fn run_single(&mut self, device: DeviceName, align: u8, erased_val: u8) {
         warn!("Running on device {} with alignment {}", device, align);
 
-        let run = ImagesBuilder::new(device, align, erased_val);
+        let run = match ImagesBuilder::new(device, align, erased_val) {
+            Some(builder) => builder,
+            None => {
+                warn!("Skipping device with insuffienct partitions");
+                return;
+            }
+        };
 
         let mut failed = false;