sunxi: Update early_print
sunxi: Enable zImage support and raw ramdisk support
sunxi spl: Add minimal FEL SPL (CONFIG_SPL_FEL)

Allwinner CPUs supports an USB bootmode called FEL where the USB
host can upload code via a simple USB protocol. To execute anything
bigger than a few KB the board first needsto be initialized and
that is the job of the FEL SPL.