Flashing Yocto Image using iPXE

[NiKiZe]

I'm assuming this is PCBIOS (not EFI)
in that case use:
initrd http://server/file.dd /file.dd

the extra /file.dd should make sure that a cpio archive header is created, and all files are appended together.
This is called initrd argument in the documentation, and when booting a bzImage this magic happens.

In EFI mode this magic does not work, so there you need to add CPIO head on your own.