glibc-external: dep on binutils

This is needed to have objcopy available for the packaging process.

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
external-toolchain: refactor sysroot extraction process

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Update README

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
external-toolchain: add docstrings to more methods

Signed-off-by: Christopher Larson <chris_larson@mentor.com>