build: also generate the CycloneDX SBOM in imagebuilder

build: also generate the CycloneDX SBOM in imagebuilder

The needed file '.packageinfo' for creating the CycloneDX SBOM in the
imagebuilder is available in the top directory of the imagebuilder and
not in the tmp directory.

For this reason, the creation of the CycloneDX SBOM file is not available
for the imagebuilder.

To fix this, it is now first checked whether the CycloneDX SBOM should be
built at all and then second decided by checking the IB variable where the
'.packageinfo' file is to be found.

With this change, it is now possible to create the CycloneDX SBOM also for
the imagebuilder as well.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>