Gcov: Enable Code coverage reporting over UART.

This patch provides support for generating Code coverage reports.
The prj.conf needs to enable CONFIG_COVERAGE. Once enabled, the
code coverage data dump now comes via UART.
This data dump on the UART is triggered once the main
thread exits.

Next step is to save this data dump on file. Then run
scripts/gen_gcov_files.py with the serial console log as argument.

The last step would be be to run the gcovr. Use the following cmd
 gcovr -r . --html -o gcov_report/coverage.html --html-details

Currently supported architectures are ARM and x86.

Signed-off-by: Adithya Baglody <adithya.nagaraj.baglody@intel.com>