rootfs.py: tweak _multilib_sanity_test for ipk incremental image generation
The _multilib_sanity_test installs multilib packages in a temporary
root fs, and compare with the current image to figure out duplicated
files that come from different packages.
While incremental image generation enabled and the previous image
was existed, there was an Multilib check error:
...
ERROR: Multilib check error: duplicate files tmp/work/qemux86_64-poky-
linux/core-image-minimal/1.0-r0/multilib/lib32/lib/libc.so.6 tmp/work/
qemux86_64-poky-linux/core-image-minimal/1.0-r0/rootfs/lib/libc.so.6
is not the same
...
The reason is the file in the existing image has been prelinked by
previous image generation and the file in a temporary root fs is not
prelinked, even though both of them came from the same package, the
Multilib check failed.
