package.bbclass: Reverse runtime symlinks should be tied to package generation

In case a package is not generated (is empty and does not has allow
empty flag set) the package data regarding reverse runtime dependency
shouldn't be done.

This were causing a false-positive in the meta-fsl-arm layer, when
building mesa, as:

,----[ Error during build of MX53 in meta-fsl-arm ]
| ERROR: The recipe mesa is trying to install files into a shared area
|  when those files already exist. Those files and their manifest
|  location are:
|    /.../build/build/tmp/sysroots/imx53qsb/pkgdata/runtime-reverse/libopenvg-dev
|    Matched in manifest-imx53qsb-amd-gpu-x11-bin-mx51.packagedata
| Please verify which recipe should provide the above files.
`----

Fixes [YOCTO: #6795]

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>