Support autoboot feature in uboot for fastmodel

version info:
        commit hash: e6bf18dba2a21bebf2c421b1c2e188225f6485a1
        HYP: khypervisor root directory
    build:
        1. download u-boot as submodule in khypervisor
                git clone git://git.denx.de/u-boot.git
        2. enter u-boot directory
                cd $(HYP)/u-boot/u-boot
        3. change the commit
                git checkout e6bf18dba2a21bebf2c421b1c2e188225f6485a1
        4. apply patch for fast model
                patch -p1 < ../../patch/u-boot_fastmodel.patch
        5. build u-boot for fast model
                make vexpress_ca15_tc2
    usage:
        see "Testing Hypervisor Prototype 2 Manually with u-boot" part in  $(HYP)/README.md