RFR: JDK-8216278: Fix devkit and basic Jib support on WSL

[Erik Joelsson]

The WSL support in the build does not currently work with devkits. This 
prevents us from using Jib to configure a WSL build. This patch fixes 
configure to work with devkits when building for Windows in WSL.

(For Oracle users, I also managed to get Jib working in WSL, and able to 
configure both Windows and Linux profiles. Note that since Jib is not 
WSL aware itself, you need to manually direct the data dir to a location 
visible from Windows. e.g --with-data-dir 
/mnt/c/cygwin64/var/tmp/jib-$USER).

Bug: https://bugs.openjdk.java.net/browse/JDK-8216278

Webrev: http://cr.openjdk.java.net/~erikj/8216278/webrev.01/index.html

/Erik