doc: Add the section Customize Parameter

* Explain how to customize the FVP parameters.
* Describe the build environment variables.
* Use repo_links when appropriate.

Signed-off-by: Qi Feng <qi.feng@arm.com>
Signed-off-by: Robbie Cao <robbie.cao@arm.com>
Issue-Id: SCM-4874
Change-Id: I8204bef047e2145057e4243c7e02fa4bb9d13d88