[OE-core] [PATCH 1/2] uboot-sign: Refactor do_deploy prefunc to do_deploy_prepend

[Daniel Klauer]

> That commit message isn't correct.
>
> It removes cleandirs for the prefunc at the start of the prefunc and
> cleandirs for the main function at the start of that main function.

Oh ok, so the prefuncs/postfuncs can have their own [cleandirs], that's
interesting.

How about the following (trying to find a good way to explain it):

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When inherited by the u-boot recipe (UBOOT_PN), uboot-sign.bbclass adds
a concat_dtb step, which places additional files into ${DEPLOYDIR}
before do_deploy.

The use of prefuncs for this prevents us from adding
  do_deploy[cleandirs] = "${DEPLOYDIR}"
because that would remove the files produced by the prefunc.
(Each of prefuncs/task/postfuncs functions has their own cleandirs, and
each function's cleandirs are removed at the start of that function.)

Thus, it seems good to make concat_dtb a part of do_deploy, such that it
runs after removal of do_deploy's cleandirs. As before, care is taken to
not interfere with the kernel's do_deploy definition, since concat_dtb
was only needed for u-boot.
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