common/cmd_fitupd.c: restore corrupted file

This file got corrupted by the automatic editin of commit 1a459660 "Add
GPL-2.0+ SPDX-License-Identifier to source files"; restore the
opiginal content and manually insert the SPDX ID.

The bug was detected by running cppcheck, which reported:
[common/cmd_fitupd.c:8]: (error) Invalid number of character ({) when
these macros are defined: 'CONFIG_UPDATE_TFTP'.

Signed-off-by: Wolfgang Denk <wd@denx.de>