RFR: JDK-8189263 Introduce CUSTOM_ROOT

[Magnus Ihse Bursie]

To support the use case where the OpenJDK repository is included as part 
of a custom project, this patch introduces the concept of a "custom 
root", which plays the role of $TOPDIR in some circumstances. It is the 
responsibility of any custom code to keep this value and propagate it 
where needed.

Bug: https://bugs.openjdk.java.net/browse/JDK-8189263
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8189263-introduce-CUSTOM_ROOT/webrev.01

/Magnus