Build: Refine Kconfig system

 - Set SFN model as default.
 - Turn off all secure partitions and refine dependencies.
 - Make regression tests imply secure partitions.
 - Check platform path and refine python excution path.
 - Other minor refinements about Kconfig options.

Signed-off-by: Jianliang Shen <jianliang.shen@arm.com>
Change-Id: I8074d02088cfe0cd93a4a1d81e12b071c26c287e
10 files changed