White shield border for Irish N-road and British A-road

[Blijbol]

Irish N-roads and British A-roads, both with yellow-on-green shields, generally do not seem to have any shield border within destination signs. However, signs suggest that if there is a border, it should be white:

• Irish N-road sign sample
• British A-road sign sample

The shields on Wikimedia Commons also have white borders:

• Irish N-road shields
• British A-road shields

This PR therefore changes the shield border colour from yellow to white.

[ZeLonewolf]

The white outline doesn't look as nice as the yellow one, but I have to agree it's an accurate representation of the real world. So, it's a yes for me unless there's a convincing reason otherwise.