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

[Erik Joelsson]

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.

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Commit messages:
 - Use target var in recipe
 - Move readme creation to its own target

Changes: https://git.openjdk.java.net/jdk/pull/1635/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1635&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257789
  Stats: 8 lines in 1 file changed: 5 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1635.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1635/head:pull/1635

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