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 <yamada.masahiro@socionext.com>
1 file changed