cmake: allow chainloading of the toolchain file

Use path from CMAKE_CURRENT_LIST_FILE to load the cmake subscripts.
This allows the toolchain file to be chainloaded from another toolchain file.

Signed-off-by: Samuli Piippo <samuli.piippo@qt.io>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>