hikey: README: Update documentation to use boot.img instead of boot_fat.uefi.img

Update the README file to use the boot.img output rather then
the boot_fat.uefi.img

Change-Id: Ice7cc25925c7865d5962c1c5cbbac29ce947a621
Signed-off-by: John Stultz <john.stultz@linaro.org>
1 file changed