Due to historical reasons, we have had two variables that points to the
output directory (e.g. "$topdir/build/linux-x64"), one named
BUILD_OUTPUT and one named OUTPUT_ROOT.
First of all, these two should be unified into one.
Second, it should be renamed OUTPUTDIR to better align with other
variables such as TOPDIR, and the various *_OUTPUTDIR variables we
already have.
The only non-trivial part of this patch was the handling in
bootcycle-spec.gmk.in. I've verified that I can still build
bootcycle-images.
Bug: https://bugs.openjdk.java.net/browse/JDK-8187544
WebRev:
http://cr.openjdk.java.net/~ihse/JDK-8187544-introduce-OUTPUTDIR/webrev.01
/Magnus
