Add incremental hash to FirmwareUpdater

This adds support for computing any hash over the update in the dtu area by providing a closure to the hash update function.

embassy-boot/boot/src/lib.rs

@@ -308,7 +308,7 @@ mod tests {
             &mut flash,
             &public_key.to_bytes(),
             &signature.to_bytes(),
-            firmware_len,
+            firmware_len as u32,
             &mut aligned,
         ))
         .is_ok());