scripts/feed: no warn on toolchain/linux overwrite

The recent 7f285d "scripts/feeds: warn when skipping core package
override" floods SDK output with warning of overwriting "linux" and
"toolchain" core packages. This should be ignored as these are not
regular packages added via feeds.

While at it slightly improve the warning string.

Signed-off-by: Paul Spooren <mail@aparcar.org>

scripts/feeds

@@ -537,7 +537,9 @@ sub install_src {
 	my $override = 0;
 	if (is_core_src($name)) {
 		if (!$force) {
-			warn "Not overriding core package $name; use -f to force\n";
+			if ($name ne "toolchain" && $name ne "linux") {
+				warn "WARNING: Not overriding core package '$name'; use -f to force\n";
+			}
 			return 0;
 		}
 		$override = 1;