RFR: 8026144: Missing mkdir in Images.gmk

[Tim Bell]

Hi Erik:

> I just hit this race when building in jprt. Adding the standard mkdir 
> -p lines should fix it.
>
> https://bugs.openjdk.java.net/browse/JDK-8026144
>
> http://cr.openjdk.java.net/~erikj/8026144/webrev.jdk.01

Good catch.  Looks good.

Tim