package_manager.py: create index files for all backends in PACKAGE_CLASSES
The previous implementation was checking IMAGE_PKGTYPE and created the
index files just for the backend used to create the image. Apparently,
'bitbake package-index' should attempt to create the index files for all
backends specified in PACKAGE_CLASSES.
[YOCTO #5827]
Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
