uninative.bbclass: Fix broken symlink issue

uninative.bbclass: Fix broken symlink issue

If two builds are sharing the same DL_DIR, and the uninative file is local
to a layer.  When the first build gets to uninative it creates the link local
to itself, and subsequent users can use the same link.  However if that first
build then is deleted from the disk, the symlink is no longer valid (broken).

We need to update the system to detect this case, and use the model
implemented by the bitbke fetch2 code.  Look for a broken link, remove it,
then try to create the link and ignore an exception if it already exists
(since we just unlinked any bad one).

(From OE-Core rev: bfd9664edad7044b5da53fc33b8d0f6508f00950)

Signed-off-by: Mark Hatle <mark.hatle@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>