pkg-cmake.mk: set CMAKE_SYSROOT variable in toolchainfile.cmake

pkg-cmake.mk: set CMAKE_SYSROOT variable in toolchainfile.cmake

This change enforces the CMAKE_SYSROOT value set in the toolchainfile.cmake.

This fix overrides the CMake heuristics used to guess it, and turns off some
non-desirable behavior adding "-isystem ..." flags to the compiler command
line, misleading the compiler and making the build failed due to some
unfound standard headers.

Fixes:
  http://autobuild.buildroot.net/results/f7e/f7e92678e91a6cb15ccf32d4a7d75b39f49d6000/defconfig
  (and others)

Cc: Ben Boeckel <mathstuf@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>