PicoPET: toolchain helper scripts

Created by: PicoPET

The purpose of this pull request (PR) is to provide members involved in Core-V development with means of building well-defined compiler toolchains for HW verification. The PR introduces scripts for fetching toolchain source code from servers external to core-v-sw repository and for building the toolchains from source, together with two examples of configuration files.

These "external" servers can be any Git servers visible from a member's environment, either publicly available (as in the examples provided), or internal to the member's organization.

File toolchain/README.md explains in detail the purpose, the structure and the usage of the scripts, and lists the enhancements envisioned at this point.

Last but not least: The location of scripts proposed in the pull request is by no means definitive. Feel free to recommend/define a better one, so that the scripts can settle there for the foreseeable future.