kunit: Rename 'kunitconfig' to '.kunitconfig'

This commit renames 'kunitconfig' to '.kunitconfig' so that it can be
automatically ignored by git and do not disturb people who want to type
'kernel/' by pressing only the 'k' and then 'tab' key.

Signed-off-by: SeongJae Park <sjpark@amazon.de>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Tested-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>