CMake: greentea: Set variables usually defined by external apps
CMake: greentea: Set variables usually defined by external apps

We rely on external applications to set MBED_CONFIG_PATH and include
app.cmake. Mbed OS can't build for a target without those parameters
set. When building greentea tests there is no external application. So,
we define the appropriate settings in our root CMakeLists.txt for
greentea tests.

mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed