[util] Add update support to get-toolchain.py
[util] Add update support to get-toolchain.py

Add support to util/get-toolchain.py to force toolchain updates on
target directory if the version tag stored in the
/toolchain_dir/buildinfo file doesn't match the version specified by the
user.

If the latest version is selected, use the version tag associated with
it when update mode is enabled.

Using the update flag:

```console
$ util/get-toolchain.py -t /tmp/riscv -r 20190827-2 --update
Downloading toolchain from <URL ...>
Toolchain downloaded and installed to /tmp/riscv
$ util/get-toolchain.py -t /tmp/riscv -r 20190827-2 --update
Toolchain version 20190827-2 already installed at /tmp/riscv. Skipping install.
```

If target dir contains a different version:

```console
$ util/get-toolchain.py -t /tmp/riscv -r 20190827-2 --update
$ Cleaning target_dir before attempting install.
$ ...
$ Toolchain downloaded and installed to /tmp/riscv
```

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