packagedata.py: Fix get_subpkgedata_fn for multilib

packagedata.py: Fix get_subpkgedata_fn for multilib

This happens when tryng to add libgcc-dev to as a multilib package
(e.g. IMAGE_INSTALL_append = " lib32-libgcc-dev")

| Processing task-core-boot...
| Processing fman-ucode...
| Processing dosfstools...
| Processing lib32-libgcc-dev...
| Unable to find package lib32-libgcc-dev (libgcc-dev)!
NOTE: package fsl-image-full-1.0-r1.1.3.6: task do_rootfs: Failed

RPM (or bitbake?) is looking in the tmp/pkgdata, however some of these file
paths are mungned for the multilib scenario:

$ find tmp/pkgdata/ | grep libgcc-dev$
tmp/pkgdata/ppce5500-fsl-linux/runtime/lib32-libgcc-dev
tmp/pkgdata/ppc64e5500-fsl-linux/runtime/libgcc-dev

This patch fixes where we look for these files so they can be found and
properly installed for the multilib root file system

Signed-off-by: Matthew McClintock <msm@freescale.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>