unconditionally apply SELinux labels to symlinks

At the end of the OTA script, we walk through /system, updating
all the permissions on the filesystem, including the UID, GID,
standard UNIX permissions, capabilities, and SELinux labels.

In the case of a symbolic link, however, we want to skip most of
those operations. The UID, GID, UNIX permissions, and capabilities
don't meaningfully apply to symbolic links.

However, that's not true with SELinux labels. The SELinux label on
a symbolic link is important. We need to make sure the label on the
symbolic link is always updated, even if none of the other attributes
are updated.

This change unconditionally updates the SELinux label on the symbolic
link itself. lsetfilecon() is used, so that the link itself is updated,
not what it's pointing to.

In addition, drop the ENOTSUP special case. SELinux has been a
requirement since Android 4.4. Running without filesystem extended
attributes is no longer supported, and we shouldn't even try to handle
non-SELinux updates anymore. (Note: this could be problematic if
these scripts are ever used to produce OTA images for 4.2 devices)

Bug: 18079773
Change-Id: I87f99a1c88fe02bb2914f1884cac23ce1b385f91