added missing instruction, resolves #728

[jancumps]

In azure-iot-device-ecosystem/get_started/raspbian-linux-optiga-tpm-slb-9670-iridium-board-c.md, there is a step missing to create the cmake directory.

Point 2: missing: mkdir azure-iot-sdk-c/cmake

before:

cd azure-iot-sdk-c/cmake
cmake -Duse_prov_client:BOOL=ON ..
cd provisioning_client/tools/tpm_device_provision
make
sudo ./tpm_device_provision

[ghost]

All CLA requirements met.