UBoot needs the kernel and ramdisk to be post processed
using mkimage utility. This element helps user do it as
part of the DIB process.
This element needs u-boot-tools to be installed.
The load address and entry point for UBoot kernel can
be specified as shown in the example below.
Example:
export UBOOT_KERNEL_ADDR=0x80000
export UBOOT_KERNEL_EP=0x80000
Change-Id: I0e1039bf57d1ef11a3f9831d82e24b1ec2136c13
