toolchainfile.cmake: only search the sysroot for CMake module

This change prevents CMake from searching outside the sysroot location
for CMake modules when cross-compiling.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>