riscv: sifive: unleashed: Add genimage config files

This adds genimage [1] config files for generating SD card and spi-nor
images, which can be programmed to an SD card or SPI flash and boot
from there.

The same images will be used for U-Boot CI testing for this board.

[1] https://github.com/pengutronix/genimage



Signed-off-by: Bin Meng <bmeng.cn@gmail.com>