Improve code coverage functionality

Code coverage tools used in the project has a strong limitation for location of source code used to compile with coverage. This source code must be in exactly same absolute path when running coverage processing tool. It's not easy to guarantee when several GitLab runners are used in project CI as these have different paths for source code location. Improve coverity pipeline where source code, execution and report generation could be in different folders.

Signed-off-by: Anton Bondarenko anton.bondarenko@arm.com