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>