scripts/populate: properly locate the sysroot and required tools

This fixes two problems:
- the sysroot might be in a sub-directory (think SYSROOT_DIR_PREFIX)
- it is not needed to have the target tuple to properly detect the sysroot
and the required tools

As a side effect, this script is now no longer dependent on the target
tuple, and in the future, we might be able to share it across many
toolchains (when/if we can install all of them in the same place).

 source config/global.in

 source config/target.in

 source config/toolchain.in

 source config/kernel.in

 source config/binutils.in

 source config/cc.in

 source config/libc.in

 source config/debug.in

 source config/companion_libs.in

 source config/companion_tools.in