kunit: tool: make `build` subcommand also reconfigure if needed

If I created a kunitconfig file that was incomplete, then
$ ./tools/testing/kunit/kunit.py build --kunitconfig=my_kunitconfig
would silently drop all the options with unmet dependencies!

This is because it doesn't do the config check that `kunit.py config`
does.

So if I want to safely build a kernel for testing, I have to do
$ ./tools/testing/kunit/kunit.py config <flags>
$ ./tools/testing/kunit/kunit.py build <flags, again>

It seems unlikely that any user of kunit.py would want the current
`build` semantics.
So make it effectively do `kunit.py config` + `kunit.py build`.

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