imgtool: Fixed missing dependency to 'pyyaml' (for dumpinfo)

imgtool's dumpinfo depends to pyyaml package, so add it to
requirements.

Signed-off-by: Denis Mingulov <denis@mingulov.com>

scripts/imgtool.nix

@@ -21,6 +21,7 @@ let
       python37.pkgs.intelhex
       python37.pkgs.setuptools
       python37.pkgs.cbor2
+      python37.pkgs.pyyaml
     ]
   );
 in

scripts/setup.py

@@ -18,6 +18,7 @@ setuptools.setup(
         'intelhex>=2.2.1',
         'click',
         'cbor2',
+        'pyyaml',
     ],
     entry_points={
         "console_scripts": ["imgtool=imgtool.main:imgtool"]