cmake: fix compiling some C++ projects with Yocto SDK and GCC

cmake: fix compiling some C++ projects with Yocto SDK and GCC

Setting CMAKE_SYSROOT in the toolchain file allows CMake to correctly
remove user-provided system include directories pointing to
<sysroot>/usr/include. The mentioned projects failed with "stdlib.h:
No such file or directory #include_next <stdlib.h>".

Signed-off-by: Urs Fässler <urs.fassler@bbv.ch>
Signed-off-by: Raphael Freudiger <raphael.freudiger@siemens.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>