add command fitupd to run an update from a FIT image

Command calls update_tftp() analogous to automatic update described
in doc/README.update.

Usage:
fitupd [addr]
        - run update from FIT image at addr
          or from tftp 'updatefile'

Signed-off-by: Andreas Pretzsch <apr@cn-eng.de>
diff --git a/doc/README.update b/doc/README.update
index 48f03b7..a7f4d9e 100644
--- a/doc/README.update
+++ b/doc/README.update
@@ -51,6 +51,11 @@
 to be prepared. Refer to the doc/uImage.FIT/ directory for more details on FIT
 images.
 
+This mechanism can be also triggered by the commmand "fitupd".
+If an optional, non-zero address is provided as argument, the TFTP transfer
+is skipped and the image at this address is used.
+The fitupd command is enabled by CONFIG_CMD_FITUPD.
+
 
 Example .its files
 ------------------