RFR: JDK-823797: ,Package type for runtime image on macosx

[Andy Herrick]

Please review fix for issue [1] at [2].

The change enables packaging runtime images as a dmg on MacOS.

[1] - https://bugs.openjdk.java.net/browse/JDK-8237971

[2] - http://cr.openjdk.java.net/~herrick/8237971/webrev.01/

/Andy