RFR: JDK-8197571 Change storage location for generated-configure.sh

[Magnus Ihse Bursie]

In JDK-8195689, the generated-configure.sh was no longer checked in, but 
locally generated. The selected location for generation ($TOPDIR/.build) 
was not unproblematic for some use cases. This patch attempts remedy this.

The new behaviour will be this:
  * If run from $TOPDIR, the storage directory will be 
$TOPDIR/build/.configure-support.
  * If run from $CUSTOM_ROOT, the storage directory will be 
$CUSTOM_ROOT/build/.configure-support.
  * If run from any other directory (about to become the build output 
directory for the configuration), the storage directory will be 
$PWD/configure-support.

This will allow "rm -rf $TOPDIR/build" to function as before to remove 
all build artifacts. It will allow configuration created in out-of-tree 
directories to have the script generated locally.

I could not put the output file in build/$BUILD/configure-support, since 
the $BUILD name is not yet determined. I did not want to put it in 
build/configure-support, since that would make it look like a 
configuration to the code that enumerates configurations in build.

I hope this addresses all issues that has been raised.

Bug: https://bugs.openjdk.java.net/browse/JDK-8197571
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8197571-change-storage-location-for-generated-configure/webrev.01

/Magnus