[U-boot] Support autoboot on 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