RFR: JDK-8004842 Unify values of boolean make variables set in configure to true/false

[Magnus Ihse Bursie]

Currently there are three different ways of expressing a boolean value 
in spec.gmk, true/false, yes/no and 1/0. We should only have one and 
since true/false is the most common one, we should stick to that.

Since this bug was opened, most cases has been fixed but two remains:
USE_PRECOMPILED_HEADER=1
ENABLE_INTREE_EC=yes
Bug: https://bugs.openjdk.java.net/browse/JDK-8004842
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8004842-standardize-booleans-in-build/webrev.01

/Magnus