board: helios-4: add config fragment for spi booting

Add a config fragment with required differences for booting from spi
flash instead of sd-card (default).

Settings for environment location are based on vendor u-boot:
https://github.com/kobol-io/u-boot/blob/helios4/include/configs/helios4.h#L59



The fragment can be applied on top of helios4_defconfig by make:
make helios4_defconfig spiboot.config

Signed-off-by: Josua Mayer <josua@solid-run.com>