I don't think there is a boot.img for that, it may be used via chroot though. Good luck on that!
Since you always replace the same values (ie. yaffs2 to ext4, a specific nand partition to a specific sd partition) what about creating a patcher, that way you wouldn't need to create a separate boot.img for every single rom.
You mean like an sh script that will unyaffs, change values, thencreate the ext img?
Edit: ignore me
Edited by razzmataz1478, 09 August 2012 - 02:56 PM.