package.bbclass: Prevent perform_packagecopy from removing /sysroot-only

package.bbclass: Prevent perform_packagecopy from removing /sysroot-only

The files in /sysroot-only are intended to make it into the
recipes sysroot output, but not into the package. However, if
do_package is run before do_populate_sysroot, the files are
removed.

Use a smaller hammer to avoid copying the files into the package so
they are still around when do_populate_sysroot runs.

Signed-off-by: Russ Dill <russ.dill@nikolamotor.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>