uboot will load "/boot/uImage" in the sd card first ext2 partition.
then kernel will also boot to rootfs at sd card first ext2 partition.
Signed-off-by: Xiangfu Liu <xiangfu@sharism.cc>
* when press [S], enable the UART0 and UART1, uboot will delay 3 second wait input from serial console.
* when press [S] + [M] + [POWER], will enalbe uart then boot from sd card.
Signed-off-by: Xiangfu Liu <xiangfu@sharism.cc>