Mark non-used functions in overwrite only mode

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/sim/src/lib.rs b/sim/src/lib.rs
index 8e394ea..2b240db 100644
--- a/sim/src/lib.rs
+++ b/sim/src/lib.rs
@@ -667,11 +667,13 @@
         fails > 0
     }
 
+    #[cfg(not(feature = "overwrite-only"))]
     fn trailer_sz(&self) -> usize {
         c::boot_trailer_sz(self.align) as usize
     }
 
     // FIXME: could get status sz from bootloader
+    #[cfg(not(feature = "overwrite-only"))]
     fn status_sz(&self) -> usize {
         self.trailer_sz() - (16 + 24)
     }