package/linux-firmware: fix special cases of symlinks

package/linux-firmware: fix special cases of symlinks

Some symlinks were not created correctly when installing the
Linux-firmware package. This patch fixes the support for all symlinks of
the form:

  a/foo -> bar
  a/foo -> b/bar
  a/foo -> ../b/bar

With this patch all forms of symlinks described in the WHENCE file
should be supported, whether they are in nested directories, or in
non-existing ones.

As some symlinks could be in directories that do not exist, we must
maje sure to canonicalize the path before testing the linked-to file.

We compared the symlinks installed pre-20200122 to what we have now, and
it seems we're handling all of them with this patch.

Fixes: 55df4059d24b ("package/linux-firmware: fix symlink support")
Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
[yann.morin.1998@free.fr:
  - use readlink in canonicalize-missing mode, to avoid
    creating-then-removing directories
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Tested-by: Antoine Tenart <antoine.tenart@bootlin.com>
Reviewed-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>