RFR: JDK-8030794: Update configure to require jdk8 as boot

[Erik Joelsson]

Since we now have dependencies in the langtools source on jdk8 features, 
we can no longer build jdk9 with jdk7 as boot. This change updates 
configure to allow jdk8 and jdk9 as boot jdks.

Bug: https://bugs.openjdk.java.net/browse/JDK-8030794
Webrev: http://cr.openjdk.java.net/~erikj/8030794/webrev.root.01/
/Erik