download-toolchain-sources.sh: Add --git-base option

Simplify the script and add a --git-base option used to
specify a new root for all toolchain repositories. This is
useful to work with custom repo installation of the toolchain
sources.

Also simplify the script.

Change-Id: Ia51613b1f3cf34c9dbeaf19f24c563c54694a350
1 file changed