Fix overwrite-only encrypted images in sim

Also add overwrite-only + encrypted tests to travis-ci.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/sim/src/lib.rs b/sim/src/lib.rs
index e7ad671..47d20eb 100644
--- a/sim/src/lib.rs
+++ b/sim/src/lib.rs
@@ -695,11 +695,13 @@
     }
 
     #[cfg(feature = "enc-rsa")]
+    #[cfg(not(feature = "overwrite-only"))]
     fn status_sz(&self) -> usize {
         self.trailer_sz() - (16 + 24 + 32)
     }
 
     #[cfg(feature = "enc-kw")]
+    #[cfg(not(feature = "overwrite-only"))]
     fn status_sz(&self) -> usize {
         self.trailer_sz() - (16 + 24 + 32)
     }