RFR: JDK-8066828: configure fails if it's set --with-boot-jdk to use JDK 9 modular image

[Erik Joelsson]

Hello,

Please review this fix to configure, removing the requirement that the 
bootjdk has an rt.jar. In the modular images change, we already did away 
with using the rt.jar in the boot jdk. With this fix, it becomes 
possible to use a modular image as boot jdk when building JDK 9.

Bug: https://bugs.openjdk.java.net/browse/JDK-8066828
Webrev: http://cr.openjdk.java.net/~erikj/8066828/webrev.root.01/

/Erik