usb-boot: Handle variable arguments
usb-boot: Fix for failure just after DRAM initialization.

just need to wait a little for the DRAM initialization to finish. Can't
use fel until we know it's finished or things breaks down.
felboot: Show u-boot version banner
felboot: Use sunxi_board_init directly from u-boot

as expected there was no problem to use sunxi_board_init directly
from u-boot. Failure on A13 was a timing issue in the host side
script (need to wait for DRAM init to finish before trying next
fel operation)
felboot: Simplify Makefile a bit to more closely match u-boot
felboot: Move closer to u-boot build structure, reducing source size even further
felboot: Use u-boot serial driver
felboot: Update documentation slightly
felboot: Reduce to only a linkerscript + stubs to compensate for general SPL config

The stubs is only needed because we are using objects from a full
SPL build. Even these will go away when felboot is integrated in
u-boot, reducing it to only a link script and config changes.