Sometime the root_path is dependent of the target platform and sometime
not. But sometime dependent of the build arch :/
[TODO] We should move the cross_file generation to the PlatformInfo class.
Introduce also a "NeutralEnv", a build environment independent of the
targeted platform. All `Source` now build using the neutralEnv.
Most of toolchains are also using neutralEnv except android_ndk who is
specific to a platform.
As toolchain are neutral, platform specific environment variables are now
set by the platformInfo directly instead of the toolchain.
Matthieu Gautier