1. Renames $FWDIR to $SPARK_HOME (vast majority of diff).
2. Use Spark-provided Maven.
3. Logs build flags in the RELEASE file.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #3973 from pwendell/master and squashes the following commits:

340a2fa [Patrick Wendell] HOTFIX: Minor improvements to make-distribution.sh