pico-imx6ul: Move dfu_alt_info to CONFIG_DFU_ENV_SETTINGS variable

Create CONFIG_DFU_ENV_SETTINGS to set dfu environment settings to
improve human readable code.

Signed-off-by: Fabio Berton <fabio.berton@ossystems.com.br>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Reviewed-by: Lukasz Majewski <lukma@denx.de>