COVERAGE: Fix COVERAGE_GCOV dependencies

CONFIG_COVERAGE has been incorrectly used to
change other kconfig options (stack sizes, etc)
code defaults, as well as some samples behaviour,
which should not have dependend on it.

Instead those should have depended on COVERAGE_GCOV,
which, being the one which adds special code and
temporary RAM storage for embedded targets,
require changes to many features.

When building for the native targets, all this was
unnecessary.

=> Fix the dependency.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>