RFR: JDK-8257789: Fix incremental build of test-image and bundles

[Magnus Ihse Bursie]

On Fri, 4 Dec 2020 23:01:26 GMT, Erik Joelsson <erikj at openjdk.org> wrote:

> When building the test-bundle incrementally, it always gets rebuilt. This is caused by the prepare-test-image target in TestImage.gmk, where we create a README file as part of a recipe for a PHONY target. This means that the README is always rebuilt, which in turn triggers downstream rebuilds. This is generally a bad pattern that should be avoided.

Marked as reviewed by ihse (Reviewer).

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PR: https://git.openjdk.java.net/jdk/pull/1635