RFR: JDK-8199266: Update boot and build jdk requirements in configure

[Erik Joelsson]

When we bumped the feature version from 10 to 11, we forgot to also 
update the required versions of the boot and build jdk. This is an easy 
mistake to do, so we should make sure it is easy and intuitive to do the 
update at the same time as updating the version number.

This patch changes the check for build jdk to simply compare with the 
current VERSION_FEATURE. This should always be the correct requirement.

The check for boot jdk is a bit more complicated. I initially thought we 
could calculate it automatically from VERSION_FEATURE-1, but until that 
version is actually released, we need to also allow VERSION_FEATURE-2. 
Because of this we will still require manual updating of this setting 
over time. So instead I moved the set of acceptable boot JDK versions to 
the version_numbers file. This should make it easy enough to spot and 
update when changing the DEFAULT_VERSION_FEATURE value.

I also updated the build documentation regarding boot JDKs to explain 
temporary N-2 situation. While at it, I also updated the part about 
where to get JDK binaries since you can now download OpenJDK builds 
directly from jdk.java.net.

Bug: https://bugs.openjdk.java.net/browse/JDK-8199266

Webrev: http://cr.openjdk.java.net/~erikj/8199266/webrev.01/

/Erik