FIH: Refine the usage of FIH tool

1. Make it skip the build if tfm_s.axf exists. Users can alse
   re-build the source code if necessary.
2. Add the support of certain function test

Signed-off-by: Jianliang Shen <jianliang.shen@arm.com>
Change-Id: Id99cf6d9f65da3ea7fe8c3b167cd06ac53d053b9
4 files changed