core/sdk: generate the SDK tarball ourselves

core/sdk: generate the SDK tarball ourselves

Currently, the wording in the manual instructs the user to generate a
tarball from "the contents of the +output/host+ directory".

This is pretty confusing, because taken literally, this would amount to
running a command like:

    tar cf my-sdk.tar -C output/host/ .

This creates a tarbomb [0], which is very bad practice, because when
extracted, it creates multiple files in the current directory.

What one really wants to do, is create a tarball of the host/ directory,
with something like:

    tar cf my-sdk.tar -C output host/

However, this is not much better, because the top-most directory would
have a very common name, host/, which is pretty easy to get conflict
with when it gets extracted.

So, we fix that mess by giving the top-most directory a recognisable
name, based on the target tuple, which we also use as the name of the
archive (suffixed with the usual +.tar.gz+.) We offer the user the
possibility to override that default by specifying the +BR2_SDK_PREFIX+
variable on the command line.

Since this is an output file, we place it in the images/ directory.

As some users expressed a very strong feeling that they do not want to
generate a tarball at all, and that doing so would badly hurt their
workflows [1], we actually prepare the SDK as was previously done, but
under the new, intermediate rule 'prepare-sdk'. The existing 'sdk' rule
obviously depend on that before generating the tarball.

We choose to make the existing rule to generate the tarball, and
introduce a new rule to just prepare the SDK, rather than keep the
existing rule as-is and introduce a new one to generate the tarball,
because it makes sense to have the simplest rule do the correct thing,
leaving advanced, power users use the longest command. If someone
already had a wrapper that called 'sdk' and expected just the host
directory to be prepared, then this is not broken; it just takes a bit
longer (gzip is pretty fast).

Update the manual accordingly.

[0] https://en.wikipedia.org/wiki/Tar_(computing)#Tarbomb
[1] http://lists.busybox.net/pipermail/buildroot/2018-June/thread.html#223377
    and some messages in the ensuing thread...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Wolfgang Grandegger <wg@grandegger.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Stefan Becker <chemobejk@gmail.com>
Cc: Trent Piepho <tpiepho@impinj.com>
Signed-off-by: &quot;Yann E. MORIN&quot; &lt;<a href="mailto:yann.morin.1998@free.fr" target="_blank">yann.morin.1998@free.fr</a>&gt;<br>
Reviewed-by: Stefan Becker <chemobejk@gmail.com>
Signed-off-by: &quot;Yann E. MORIN&quot; &lt;<a href="mailto:yann.morin.1998@free.fr" target="_blank">yann.morin.1998@free.fr</a>&gt;<br>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>