Rebuild issues in imaging steps with local packages

["Moessbauer, Felix" <felix.moessbauer@siemens.com>]

Dear Devs,

I just observed a nasty rebuild issue in the imaging step:

When building a custom package that is later used in the IMAGER_INSTALL
and IMAGER_BUILD_DEPS, this package does ONLY get updated on version
changes. This can be checked by inspecting log.do_install_imager_deps
and checking for a line "<package> is already the newest version".

I found this while debugging why a fresh build of the "Add BSP for
StarFiveTech VisionFive2 Risc-V board" (v1) series on the ML is broken
for another user. The reason was simply that I optimized it locally and
made changes to the visionfive2-u-boot-firmware, which were not
included in the final image.

This behavior is different from how packages in the rootfs are handled,
but I did not debug yet why this is different. Maybe Adriaan can help.
It could also well be, that this issue is automatically fixed when
moving the imaging to the schroot.

Best regards,
Felix