tests: improve code coverage support

Fix the exclusion path for lcov; it should exclude the directory
with source code, not object files.

Use the COV environment variable to
* control whether we build for coverage or not
* select the output directory

Add a separate target for generating the report, so we can get a
report for all of the tests together or just a single test.

Add documentation.

Signed-off-by: Paul Fagerburg <pfagerburg@google.com>
Change-Id: I2bd2bfdedfab291aabeaa968c10b17e9b61c9c0a
Reviewed-on: https://review.coreboot.org/c/coreboot/+/54072
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jakub Czapiga <jacz@semihalf.com>

Documentation/tutorial/part3.md

@@ -3,6 +3,8 @@
 ## Introduction
 General thoughts about unit testing coreboot can be found in
 [Unit testing coreboot](../technotes/2020-03-unit-testing-coreboot.md).
+Additionally, [code coverage](../technotes/2021-05-code-coverage.md) support
+is available for unit tests.
 
 This document aims to guide developers through the process of adding and writing
 unit tests for coreboot modules.