kunit: tool: add --kconfig_add to allow easily tweaking kunitconfigs

E.g. run tests but with KASAN
$ ./tools/testing/kunit/kunit.py run --arch=x86_64 --kconfig_add=CONFIG_KASAN=y

This also works with --kunitconfig
$ ./tools/testing/kunit/kunit.py run --arch=x86_64 --kunitconfig=fs/ext4 --kconfig_add=CONFIG_KASAN=y

This flag is inspired by TuxMake's --kconfig-add, see
https://gitlab.com/Linaro/tuxmake#examples

.

Our version just uses "_" as the delimiter for consistency with
pre-existing flags like --build_dir, --make_options, --kernel_args, etc.

Note: this does make it easier to run into a pre-existing edge case:
$ ./tools/testing/kunit/kunit.py run --arch=x86_64 --kconfig_add=CONFIG_KASAN=y
$ ./tools/testing/kunit/kunit.py run --arch=x86_64
This second invocation ^ still has KASAN enabled!

kunit.py won't call olddefconfig if our current .config is already a
superset of the provided kunitconfig.

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>