Update trailer format

This implements changes according to MCUB-14, easing the process
of making external apps parse and read/write the trailer.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/sim/src/c.rs b/sim/src/c.rs
index 4de9553..a770ca6 100644
--- a/sim/src/c.rs
+++ b/sim/src/c.rs
@@ -33,6 +33,14 @@
     unsafe { raw::sim_flash_align = align };
 }
 
+pub fn boot_magic_sz() -> usize {
+    unsafe { raw::BOOT_MAGIC_SZ as usize }
+}
+
+pub fn boot_max_align() -> usize {
+    unsafe { raw::BOOT_MAX_ALIGN as usize }
+}
+
 mod raw {
     use area::CAreaDesc;
     use libc;
@@ -46,5 +54,8 @@
 
         pub static mut sim_flash_align: u8;
         pub fn boot_slots_trailer_sz(min_write_sz: u8) -> u32;
+
+        pub static BOOT_MAGIC_SZ: u32;
+        pub static BOOT_MAX_ALIGN: u32;
     }
 }