Add CI tests for OTA:

The test provision an S3 bucket with the firmware built during
the build phase and create an OTA job for the device to be updated.
To accommodate the ML test that requires a binary not connectable
to the cloud to run efficiently, two builds are generated during
the build phase.

These requirements induced some changes on the build system:
- Location of the aws credentials can be set using the CMake flag
AWS_CONFIG_CREDENTIALS_PATH.
- Build tree can be defined for building and running.

Signed-off-by: Vincent Coubard <vincent.coubard@arm.com>