RFR: JDK-8230519: jpackage "--package-type" values and default

[Andy Herrick]

Please review the jpackage fix for bug [1] at [2].

This is a fix for the JDK-8200758-branch branch of the open sandbox 
repository (jpackage).

This fix:

Adds "app-image" as a valid value for "--package-type" options, meaning 
build an application image instead of a package.

Changes the default value of "--package-type" to a platform dependent 
default package type.

[1] https://bugs.openjdk.java.net/browse/JDK-8230519
[2] http://cr.openjdk.java.net/~herrick/8230519/webrev.01/

Thanks

Andy,