classes/package_rpm: fix bitbake package-index for RPM

classes/package_rpm: fix bitbake package-index for RPM

The function that "bitbake package-index" relies upon when using the RPM
package backend (package_update_index_rpm()) uses MULTILIB_PREFIX_LIST
to get the list of package architectures to be indexed, but that
variable is only set when populate_sdk_rpm or rootfs_rpm are inherited,
which is not the case for the package-index recipe. Until we're able to
refactor this properly, for minimal impact just use the value of
ALL_MULTILIB_PACKAGE_ARCHS if MULTILIB_PREFIX_LIST does not give us any
architectures (the equivalent function in the ipk backend uses the
former variable).

Having "bitbake package-index" working is important because it's the
only practical way of indexing RPM packages for use as a feed; host
versions of createrepo won't work properly because they won't support
indexing recommends relationships.

Stopgap fix for [YOCTO #5278].

(From OE-Core rev: 9359719c563e1ab0ff10186d1a1b6bde7840dbf3)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>