Added write_uboot_spl deviceinfo variable
This adds the write_uboot_spl
deviceinfo variable. This allows a file from inside the device chroot to be flashed to the start of the SD card with an offset of 8KB. This is used to put u-boot on the sdcard for at least allwinner devices.
An example for the PINE A64 is: write_uboot_spl="/usr/share/u-boot/pine64_plus/u-boot-sunxi-with-spl.bin"