RFR: 8026144: Missing mkdir in Images.gmk

[Erik Joelsson]

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/

/Erik