tools: env: allow to print U-Boot version

The fw_env utility family has a default environment compiled in
which ties it quite strongly to the U-Boot source/config it has
been built with. Allow to display the U-Boot version it has been
built with using the -v/--version argument.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>