The base addresses of U-Boot on different SoC are differen, add it
as parameter.
eg:
loaderimage --pack --uboot ./u-boot.bin rk3229_uboot.img 0x60000000
loaderimage --pack --uboot ./u-boot.bin rk3399_uboot.img 0x200000

Change-Id: Ib29bb5c206e0bb0a201ec24dc28fa0dd0b6e891f
Signed-off-by: Kever Yang <kever.yang@rock-chips.com>