HOTFIX: Minor improvements to make-distribution.sh

[pwendell]

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

[SparkQA]

Test build #25311 has started for PR 3973 at commit 340a2fa.

• This patch merges cleanly.

[SparkQA]

Test build #25311 has finished for PR 3973 at commit 340a2fa.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/25311/
Test PASSed.

[pwendell]

Okay pulling this into master. These are just minor clean-up's to this internal script.