[test] use cmake to build ot-rcp (#1212)
[test] use cmake to build ot-rcp (#1212)

This commit switch to cmake when building ot-rcp during tests.
openthread/ot-br-posix,openthread/borderrouter,openthread/ot-br-posix,openthread/ot-br-posix,openthread/ot-br-posix,openthread/ot-br-posix,openthread/borderrouter,openthread/borderrouter,openthread/ot-br-posix,openthread/borderrouter,openthread/borderrouter,openthread/borderrouter