automatic update from FIT image: add optional address parameter

Current update_tftp() flow:
  1.) fetch "updatefile" from defined TFTP server
  2.) check if FIT format
  3.) flash contained images

Add an address parameter to update_tftp(). If this address is non-zero,
skip the TFTP transfer and use the image at this address.
Also extend update_tftp() to return success/fail.

Signed-off-by: Andreas Pretzsch <apr@cn-eng.de>
diff --git a/common/main.c b/common/main.c
index 2124526..1f22142 100644
--- a/common/main.c
+++ b/common/main.c
@@ -51,7 +51,7 @@
 void show_boot_progress (int val) __attribute__((weak, alias("__show_boot_progress")));
 
 #if defined(CONFIG_UPDATE_TFTP)
-void update_tftp (void);
+int update_tftp (ulong addr);
 #endif /* CONFIG_UPDATE_TFTP */
 
 #define MAX_DELAY_STOP_STR 32
@@ -341,7 +341,7 @@
 #endif /* CONFIG_PREBOOT */
 
 #if defined(CONFIG_UPDATE_TFTP)
-	update_tftp ();
+	update_tftp (0UL);
 #endif /* CONFIG_UPDATE_TFTP */
 
 #if defined(CONFIG_BOOTDELAY) && (CONFIG_BOOTDELAY >= 0)