Well, if you are designing a custom bootloader, you only need to know how to
inform the kernel of the location of the DTB and the root filesystem (initrd).
You then can pack everything in the manner you prefer, and unpack and transfer
the information mentioned above when you boot the kernel. The kernel expects
a pointer to the DTB in r3, the start address of the initrd in r4 and the end address of
initrd in r5. Take a look at arch/powerpc/kernel/head_fsl_booke.S for better
understanding.
If you wish to use a combined image format that already exists, take a look at
zImage. This format can combine all three components. Study the following
for more information:
Documentation/powerpc/bootwrapper.txt
arch/powerpc/boot/wrapper
All paths above are from the top level of the kernel source directory hierarchy.
I would also recommend to study Section 5 of ePAPR to understand how the bootloader,
the device tree and the kernel interact. This document can be downloaded from here:
http://www.freescale.com/infocenter/topic/QORIQSDK/2888193.html
Have a great day,
Platon
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