twister: coverage: GCOVR as defautlt coverage reporting tool

Twister now uses GCOVR by default as the more reliable code
coverage reporting tool instead of LCOV.

Signed-off-by: Dmitrii Golovanov <dmitrii.golovanov@intel.com>

doc/develop/test/coverage.rst

@@ -142,7 +142,11 @@ or::
 
     $ twister --coverage -p native_sim -T tests/bluetooth
 
-which will produce ``twister-out/coverage/index.html`` with the report.
+which will produce ``twister-out/coverage/index.html`` report as well as
+the coverage data collected by ``gcovr`` tool in ``twister-out/coverage.json``.
+
+Other reports might be chosen with ``--coverage-tool`` and ``--coverage-formats``
+command line options.
 
 The process differs for unit tests, which are built with the host
 toolchain and require a different board::

scripts/pylib/twister/twisterlib/environment.py

@@ -305,7 +305,7 @@ structure in the main Zephyr tree: boards/<arch>/<board_name>/""")
                              "This option may be used multiple times. "
                              "Default to what was selected with --platform.")
 
-    parser.add_argument("--coverage-tool", choices=['lcov', 'gcovr'], default='lcov',
+    parser.add_argument("--coverage-tool", choices=['lcov', 'gcovr'], default='gcovr',
                         help="Tool to use to generate coverage report.")
 
     parser.add_argument("--coverage-formats", action="store", default=None, # default behavior is set in run_coverage