docs: qemu: add build instructions for QEMU_EFI.fd and rootfs.cpio.gz
docs: qemu: add build instructions for QEMU_EFI.fd and rootfs.cpio.gz

This commit solves the limitation, "No build instructions for
QEMU_EFI.fd and rootfs-arm64.cpio.gz"

Document the steps to build them.

Change-Id: Ic6d895617cf71fe969f4aa9820dad25cc6182023
Signed-off-by: Masahiro Yamada <378b411a8a63ecc7605ec4272234862d7781c6ae@socionext.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware