build: allow calling append-dtb from image build commands

build: allow calling append-dtb from image build commands

mpc85xx uses this for firmware image files, since the dtb data is not
directly part of the kernel image. This causes build failures in the
image builder.

Fix this by adding a separate build step that runs this call earlier,
reusing the generated file for any calls from kernel or image build
commands.

Signed-off-by: Felix Fietkau <nbd@nbd.name>