multilib.bbclass: fix incorrect TARGET_VENDOR in multilib image
While building multilib extended images such as libXX-core-image-minimal,
the WORKDIR has the same dir with the building of core-image-minimal.
$ ls tmp/work/qemux86_64-poky-linux/ -al
...
drwxrwxr-x 3 jiahongxu jiahongxu 4096 Oct 13 16:01 core-image-minimal
drwxrwxr-x 3 jiahongxu jiahongxu 4096 Oct 16 11:11 lib32-core-image-minimal
...
While image class is inherited, it did not assign OVERRIDES with
'virtclass-multilib-libXXX', so the reason is variable TARGET_VENDOR was
not override for multilib in that situation.
It refers what did for PN and MLPREFIX, and manually do the multilib
override for TARGET_VENDOR in RecipePreFinalise handler.
[YOCTO #6844]
Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
