Load the kernel at 0x82000000 instead of 0x80000000.

U-Boot loads at 0x80000000 so the kernel must load in a different
address to avoid conflicts. I choose 0x82000000 because omap4boot
loads the kernel image there, so we don't need usbboot/usbboot.S
anymore. omap4boot can now load kernel.bin in the PandaBoard without
any problems.