[yocto] Understanding dtb generation wrt uboot (with signing)

[colin.helliwell at ln-systems.com]

I'm working on 2.2/Morty, with a custom board.
Ultimate goal is to create a FIT image with a signed configuration. But I'm
trying to just work through step by step and understand the relationships
between u-boot, kernel, dtb, and the fit image - would appreciate any
guidance!
So far, I've built and booted a fitImage. 

It seems, however, that I need to program my modified dtb into the Flash
(i.MX6 cpu) - if I program in the manufacturer's default then the kernel
doesn't get my modified one. May be a problem for another time - I can
program mine into Flash if need be.

First thing I'm puzzled by is that the dtb *is* in
tmp/deploy/images/mymachine/ , but is called
"zImage--4.4-r0-mymachine-20170727160613.dtb". Also with a symlink
"zImage-mymachine.dtb".  (If I'm understanding things right, this is because
the fitImage contains a compressed kernel...?)

Then I set UBOOT_SIGN_ENABLE = "1", and re-bitbake the u-boot target. It
only gets as far as re-building the kernel, which fails on the
do_assemble_fitimage  stage because uboot-mkimage  is being told to use
"u-boot.dtb"

So I'm unsure what setting(s) I need to tweak to pull things into line. Why
'zImage... .dtb' being built yet 'u-boot.dtb' being looked for?