at: add sha256 for legal-info files

Copyright and COPYING file sha256 are missing.

Add them calculating locally.

Signed-off-by: Giulio Benetti <giulio.benetti@micronovasrl.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/at/at.hash

@@ -1,2 +1,5 @@
 # Locally calculated
 sha256 f5c7c8226fab0bc4e2d16a55e04d4026f3452db51fc5cbcc4bb5a3c79a79f7ef  at-release_3.1.23.tar.gz
+
+sha256 01dccc0975aa9ba1a9f83e7c5e04f16077353d3c72a0a759b8846ee7a5b2b616  Copyright
+sha256 c38aee9e3c8c4d5d594ff548a1be05453023016d6286931f6512db215ec1fd42  COPYING