kunit: tool: add support for building kernel with additional threads Allows the user to specify how many threads to be used for building the kernel as part of building and running tests. Google-Bug-Id: 131440297 Signed-off-by: Brendan Higgins <brendanhiggins@google.com> Change-Id: If1c10b7e122b5863b7e07f467c7008d3e6397336