tools/qa-checks: Fix relative path calculation to the default config file

The user should be able to execute the run-checks.py script from any location.
This patch fixes an issue where the relative path to the configuration YAML file
was not being resolved properly.

Issue-Id: SCM-3033
Signed-off-by: Richard Neill <richard.neill@arm.com>
Change-Id: Ia5ede51cf240eddb4fed436ed2ccc69d33b5bd3e