Ensure we add a new line when appending to rc.local

When we add content to rc.local, if the file already
exists then we need to make sure we add the content
after a new line explicitly

Fixes LP #1089668

Change-Id: I35be1496703b302f732363fa76ce832505eed599
(cherry picked from commit f393a513d7894ddb800e4dfc87da896600fb7421)