Fix codegen of root choices

Fix codegen of root choices

If a choice which is a direct child of a module is targeted via
an augment, codegen fails.

The reason for this is that it attempts to use an empty SchemaPath
to find a DataSchemaNode: something that cannot be done. The problem
is the assumption that a Module is a SchemaNode -- which it is not.

To fix this, recognize when the parent path is empty and short-circuit
to the defining module if that is the case.

JIRA: MDSAL-345
Change-Id: I52679c822b220271d943bfbc421049d2dc394d70
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>