kunit: tool: move Kconfig read_from_file/parse_from_string to package-level

read_from_file() clears its `self` Kconfig object and parses a config
file.

It is a way to construct Kconfig objects more so than an operation on
Kconfig objects. This is reflected in the fact its only ever used as:
  kconfig = kunit_config.Kconfig()
  kconfig.read_from_file(path)

So clean this up and simplify callers by replacing it with
  kconfig = kunit_config.parse_file(path)

Do the same thing for the related parse_from_string() function as well.

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>