doc: Update CI build tool directory

This patch updates the documentation to state that the kas-ci-build.py build
script may be found in the project's tools/build directory, and clarifies that
the YAML files provided to it must be valid and present as kas configuration
files within meta-ewaol-config/kas.

Issue-Id: SCM-3042
Signed-off-by: Richard Neill <richard.neill@arm.com>
Change-Id: I9743ab0964b89c55a60793f1996867f6e4e3efed