nativesdk-cmake: Adjust toolchain paths dynamically

This patch adds a flexible way to configure the CMake in SDKs. It adds
a toolchain configuration script which supports subscripts for
extensions, as for example Qt5.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>