(urgent) RFR: JDK-8220529: JDK-8220383 broke test image build

[Erik Joelsson]

In JDK-8220529 I managed to break the test image build. This patch fixes 
that. It's still being verified as I'm sending this.

In JDK-8220529 I moved some of the logic in JtregNative*.gmk into 
conditionals to avoid re-evaluating those expensive parts when building 
images. The problem with this was that some variables used in the images 
generation were defined in the now excluded parts of the makefiles. I 
changed the usage of these variables to instead find the files to copy 
using wildcard.

(Just doing a build comparison was not enough in this case because the 
compare script does not verify the contents of the test image. I've 
filed a followup to fix this as well)

https://bugs.openjdk.java.net/browse/JDK-8220529

http://cr.openjdk.java.net/~erikj/8220529/webrev.01/