wic: Use ROOTFS_DIR to locate pseudo_dir

wic: Use ROOTFS_DIR to locate pseudo_dir

Fixed when wks is:
part /boot --source bootimg-partition --ondisk mmcblk0 --fstype=vfat --label boot --active --align 4096 --fixed-size 256M
part / --source rootfs --exclude-path=var/ --ondisk mmcblk0 --fstype=ext4 --label root --align 4096
part /var --source rootfs --rootfs-dir=${IMAGE_ROOTFS}/var --ondrive mmcblk0 --fstype=ext4 --label var --align 4096 --fixed-size 1024M --fsoptions=ro,relatime,sync

$ bitbake <image>
$ wic create <wks> -e <image>
WARNING: /path/to/rootfs/var/../pseudo folder does not exist. Usernames and permissions will be invalid

The --rootfs-dir can be anywhere, so the ../pseudo may not exist, use
ROOTFS_DIR to fix the problem.

From: Rath Anil Kumar <AnilKumar.Rath@windriver.com>
Signed-off-by: Robert Yang <liezhi.yang@windriver.com>