tools: mkimage: Show where signatures/keys are written

At present mkimage displays the node information but it is not clear what
signing action was taken. Add a message that shows it. For now it only
supports showing a single signing action, since that is the common case.

Sample:

   Signature written to 'sha1-basic/test.fit',
       node '/configurations/conf-1/signature'
   Public key written to 'sha1-basic/sandbox-u-boot.dtb',
       node '/signature/key-dev'

Signed-off-by: Simon Glass <sjg@chromium.org>