fdt: Check for a token to skip auto-hash validation

fdt: Check for a token to skip auto-hash validation

Allow the itb file to declare to u-boot that its hash should not be
checked automatically on bootm or iminfo.  This allows an image to
either be checked automatically or to include a script which may
check it otherwise (such as after part of the itb has been relocated
to RAM by the script).

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>