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>

Approved-by: Jakub Sujak <jakub.sujak@arm.com>