scripts/feeds: do not warn if override is already installed

When attempting to install a package that is already present in core and -f is not used, suppress the warning about not overriding in case override has already been installed. This avoids spurious warnings caused by dependencies being installed, which is always done without force, or by running "scripts/feeds install -f -p -a" followed by "scripts/feeds install -a" for example.

Change-Id: If1a4926aa31e36273ca28d91d3aeba0907ee87ea