wic/rootfs.py: allow --exclude-path option to exclude symlinks

wic/rootfs.py: allow --exclude-path option to exclude symlinks

Currently, if we specify a symbolic link in --exclude-path option,
we will get the following error in do_image_wic:

ERROR: --exclude-path: Must point inside the rootfs: usr/bin/hello.link

This is because it uses os.path.realpath to eliminate symbolic links. To
exclude symbolic links, use os.path.abspath instead of os.path.realpath.

Signed-off-by: Yi Zhao <yi.zhao@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>