sim: Get header magic value from ManifestGen trait

Different manifest objects will use different magic values for the
header.  Allow this support by providing a query to retrieve the
appropriate magic value.

Signed-off-by: David Brown <david.brown@linaro.org>
diff --git a/sim/src/image.rs b/sim/src/image.rs
index 3404ddd..e673807 100644
--- a/sim/src/image.rs
+++ b/sim/src/image.rs
@@ -668,7 +668,7 @@
 
     // Generate a boot header.  Note that the size doesn't include the header.
     let header = ImageHeader {
-        magic: 0x96f3b83d,
+        magic: tlv.get_magic(),
         load_addr: 0,
         hdr_size: HDR_SIZE as u16,
         _pad1: 0,