tools/image-info: fix warning when running with python 3.8

Python 3.8 introduced this warning:

tools/image-info:53: SyntaxWarning: "is" with a literal. Did you mean "=="?
  if line[0] is '#':

This commit fixes this warning. More info in Python 3.8 release notes:
https://docs.python.org/3.8/whatsnew/3.8.html#changes-in-python-behavior

tools/image-info

@@ -50,7 +50,7 @@ def parse_environment_vars(s):
         line = line.strip()
         if not line:
             continue
-        if line[0] is '#':
+        if line[0] == '#':
             continue
         key, value = line.split("=", 1)
         r[key] = value.strip('"')