RFR: JDK-8235955: [dmg] DMG creation fails without error message if previous DMG was not ejected

[Alexander Matveev]

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

- Modified customization script for DMG to use location where DMG is 
mounted to identify image that we need to customize, instead of using 
name which might not be unique.

[1] https://bugs.openjdk.java.net/browse/JDK-8235955

[2] http://cr.openjdk.java.net/~almatvee/8235955/webrev.00/

Thanks,
Alexander