doc: Add instructions for building an N1SDP image with tests using build script

Instructions are given for building FVP images with validation tests using kas
directly and using the tools/build/kas-ci-build.py build script. This patch
makes the documentation consistent by also stating both methods for the N1SDP.

Issue-Id: SCM-3553
Signed-off-by: Richard Neill <richard.neill@arm.com>
Change-Id: I7b0a129d24701fd84a59ba17fcbca389a1f85e70