atftp: add hash file

Adding a hash file will allow autobuilders to detect that the files
they have downloaded is invalid. It will be removed, and then
re-downloaded in a followup build.

Should fix issues such as:

  http://autobuild.buildroot.org/results/939/93963514fc42d481808b1a046f3e1376286bdae2/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>