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.

usb-boot

@@ -35,6 +35,7 @@ if [ ! -f $bootscr ]; then
 fi
 fel write 0x2000 $felboot
 fel exe 0x2000
+sleep 1 # Wait for DRAM initialization to complete
 if [ -n "$uboot" ]; then
 	fel write 0x4a000000 $uboot
 fi