rboot: Add cryptographic digest support for OTA images & SHA256 example

extras/rboot-ota/rboot-api.h

@@ -144,6 +144,33 @@ uint32_t rboot_get_slot_offset(uint8_t slot);
 **/
 bool rboot_verify_image(uint32_t offset, uint32_t *image_length, const char **error_message);
 
+
+/* @description Digest callback prototype, designed to be compatible with
+   mbedtls digest functions (SHA, MD5, etc.)
+
+   See the ota_basic example to see an example of calculating the
+   SHA256 digest of an OTA image.
+*/
+typedef void (*rboot_digest_update_fn)(void * ctx, void *data, size_t data_len);
+
+/** @description Calculate a digest over the image at the offset specified
+
+    @note This function is actually a generic function that hashes SPI
+   flash contents, doesn't know anything about rboot image format. Use
+   rboot_verify_image to ensure a well-formed OTA image.
+
+   @param offset - Starting offset of image to hash (should be 4 byte aligned.)
+
+   @param image_length - Length of image to hash (should be 4 byte aligned.)
+
+   @param update_fn - Function to update digest (see rboot_digest_update_fn for details)
+
+   @param update_ctx - Context argument for digest update function.
+
+   @return True if digest completes successfully, false if digest function failed part way through
+**/
+bool rboot_digest_image(uint32_t offset, uint32_t image_length, rboot_digest_update_fn update_fn, void *update_ctx);
+
 #ifdef __cplusplus
 }
 #endif