[settings] Fix setting_cmp() to handle nameless settings

[settings] Fix setting_cmp() to handle nameless settings

setting_cmp() compares by option tag and then by name.  Empty names
will always match, which gives us a false positive.

Fix by explicitly checking for empty names.

Modified-by: Michael Brown <mcb30@etherboot.org>
Signed-off-by: Michael Brown <mcb30@etherboot.org>