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).

 # MIPS specific config options

 config ARCH_mips

     select ARCH_SUPPORTS_32

     select ARCH_SUPPORTS_64 if EXPERIMENTAL

     select ARCH_DEFAULT_32

     select ARCH_USE_MMU

     select ARCH_SUPPORTS_BOTH_ENDIAN

     select ARCH_DEFAULT_BE

     select ARCH_SUPPORT_ARCH

     select ARCH_SUPPORT_ABI

     select ARCH_SUPPORT_TUNE

     help

       The MIPS architecture, as defined by:

         http://www.mips.com/