[yocto] How to build Yocto zImage ramdisk image format

[JH]

Hi,

I build a zImage--imx.bin, dose it contains initramfs? I downloaded
the zImage to IMX using MfgTool, it got an error "Wrong Ramdisk Image
Format".

How to build a ramdisk image? Should it be a separate command or
should it use a ramdisk recipe?

I found a couple of initramfs.bb, such as
fsl-image-mfgtool-initramfs.bb, core-image-minimal-initramfs.bb, but
there is no ramdisk.bb.

Appreciate any examples to build a ramdisk image format.

Thank you.

- JH