scripts/feeds: warn when skipping core package override

Otherwise, a n00b like myself can get quite confused when moving a
package from core to feeds, for example.

(Hint: one *really* needs to clear out the tmp/info/.packageinfo...
entries for the stale package, but '-f' works as well.)

Signed-off-by: Brian Norris <computersforpeace@gmail.com>

scripts/feeds

@@ -536,7 +536,10 @@ sub install_src {
 	# If it's a core package and we don't want to override, just return
 	my $override = 0;
 	if (is_core_src($name)) {
-		return 0 unless $force;
+		if (!$force) {
+			warn "Not overriding core package $name; use -f to force\n";
+			return 0;
+		}
 		$override = 1;
 	}