RFR: JDK-8170528: Race condition with release file creation

[Erik Joelsson]

The release file is part of all product images in the build. We used to 
generate it for each image individually, but a while back, we changed to 
generate it just for the exploded image, and then let jlink use the one 
as source for generating individually modified release files for each 
linked image.

The problem is that the release file is still generated in Images.gmk, 
which is called multiple times. This means each call to this file may 
try to generate the release file at the same time, which may result in a 
corrupt release file.

This patch moves the creation of the base release file to a separate 
build step.

Bug: https://bugs.openjdk.java.net/browse/JDK-8170528

Webrev: http://cr.openjdk.java.net/~erikj/8170528/webrev.01

/Erik