perf(core): optimize boot speed by checking empty image area by words

[no changelog]

core/embed/lib/image.c

@@ -280,23 +280,42 @@ secbool check_image_contents(const image_header *const hdr, uint32_t firstskip,
       return secfalse;
     }
 
+    uint32_t expected_word = expected_byte << 24 | expected_byte << 16 |
+                             expected_byte << 8 | expected_byte;
+
     while (offset < end_offset) {
       size_t bytes_to_check = MIN(
           IMAGE_CHUNK_SIZE - (offset % IMAGE_CHUNK_SIZE), end_offset - offset);
+      size_t words_to_check = bytes_to_check / sizeof(uint32_t);
+      size_t single_bytes_to_check = bytes_to_check % sizeof(uint32_t);
+
+      const uint8_t *bytes = (const uint8_t *)flash_area_get_address(
+          area, offset, single_bytes_to_check);
+      if (!bytes) {
+        return secfalse;
+      }
+
+      for (size_t i = 0; i < single_bytes_to_check; i++) {
+        if (bytes[i] != expected_byte) {
+          return secfalse;
+        }
+      }
+
+      offset += single_bytes_to_check;
 
-      const uint8_t *data =
-          (const uint8_t *)flash_area_get_address(area, offset, bytes_to_check);
+      const uint32_t *data = (const uint32_t *)flash_area_get_address(
+          area, offset, bytes_to_check - single_bytes_to_check);
       if (!data) {
         return secfalse;
       }
 
-      for (size_t i = 0; i < bytes_to_check; i++) {
-        if (data[i] != expected_byte) {
+      for (size_t i = 0; i < words_to_check; i++) {
+        if (data[i] != expected_word) {
           return secfalse;
         }
       }
 
-      offset += bytes_to_check;
+      offset += words_to_check * sizeof(uint32_t);
     }
   }