ci: Add top-level CI configuration

This commit adds the top-level CI configuration for the project, which
extends the baseline configuration and adds the "build" and "test"
stages. During the "build" stage, the Blinky example is built for the
various different configurations of the project. Then, in the "test"
stage, the outputs of the built examples are checked.

Signed-off-by: Chuyue Luo <Chuyue.Luo@arm.com>