scripts/functions: add a save/restore handler

Saving and restoring the steps requires saving/restoring multiple
directories. Depending on the configuration, some may not exist.

Add a wrapper that checks before creating/extracting the tarballs.

reporter_name="Yann E. MORIN"

reporter_url="http://ymorin.is-a-geek.org/projects/crosstool"

reporter_comment="Experimental toolchain for ARM cortex-m3 in Thumb-only, noMMU, and newlib."