package/dahdi-linux: new package

package/dahdi-linux: new package

dahdi-linux provides kernel modules to drive a variety of telephony
cards, ranging from low-end one-channel to higher-end multi-channel
cards. It also provides headers for userland to talk to those cards.

With a bit of love, dahdi-linux can use our kernel-module
infrastructure. Wee! :-)

Still, there are a few specificities about dahdi-linux.

First, it needs to install a few binary firmware blobs, which it wants
to download at install time. Since we do want to be able to do
completely off-line builds, we need to downlaod them manually. So we
have the full list of firmware blobs (even if some can only be used on
an i386/x86_64 target, we still uconditionally download them), for which
we have locally-computed sha256 (no hash provided by upstream for the
blobs).

Second, the install procedure for the firmware blobs needs to have
access to the Linux kernel .config file, so it can decide whether to
install the blobs or not. We can force not to install them, but we can't
force to install them... :-/ And anyway, we'd have to do the same check
as is already done by dahdi-linux, so no need to duplicate that.

Finally, the licensing is relatively weird. Although it is obvious and
straightforward for the most part of dahdi-linux, consisting of mostly
GPLv2 and a few LGPLv2.1, there is one gotcha.

Of the firmware blobs, one is provided as a .o file, with no licensing
information whatsoever, without any source available from upstream, but
is directly linked to a GPLv2 file.

This is very concerning, but there is not much we can do about it,
except delegate to the legal reviewer whether that is acceptable or not.

AS an aside, dahdi-linux drivers do not build with a kernel 4.0 or
later, as it uses internals that have been removed in linux-4.0. There
has been no update upstream dahdi-linux to fix that. There's not much we
can do, except warn the user in the help text.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Arnout: use SPDX license names and add hashes for license files]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>