sunxi: board: extract creating a unique sid into a helper function

Refactor setup_environment() so we can use the created sid for a
Bluetooth address too.

Signed-off-by: Andre Heider <a.heider@gmail.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
[rebased]
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>