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).

# Compute AVR32-specific values

CT_DoArchTupleValues() {

    # The architecture part of the tuple:

    CT_TARGET_ARCH="${CT_ARCH}"

    # gcc ./configure flags

    CT_ARCH_WITH_ARCH=

    CT_ARCH_WITH_ABI=

    CT_ARCH_WITH_CPU=

    CT_ARCH_WITH_TUNE=

    CT_ARCH_WITH_FPU=

    CT_ARCH_WITH_FLOAT=

    CT_TARGET_SYS=none

}