dnf, libdnf: Ignore if PACKAGE_CLASSES does not have rpm
dnf depends on libsolv, and libsolv does not have and easy way to
jusr enable rpmdb support, it either has full rpm or nothing, perhaps it
could be improved in libsolv and we can use that but from now we have
no other option
dnf support only rpm backend and wont work with opkg or dpkg/apt anyway
Signed-off-by: Khem Raj <raj.khem@gmail.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
