Fix mergeable if-the-else statements

Fix mergeable if-the-else statements

This fixes DTOs to use simple return expressions. While we are in
the area, also optimize toString() methods to lower their footprint.

Change-Id: Ie206c2942c5fb9b6cbbdc2a31d197560559fd660
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit 18748923d6759e0e301b608e6d6bb15a69583bbb)
(cherry picked from commit f9c46078ecd09194cb9f109297c70a296f349829)