LICENSES: remove empty other, exceptions directories ...

These directories were originally added as they were required for
spdxcheck.py, which upstream checkpatch.pl invokes.

As git doesn't index empty directories, they contained empty
.gitignore files. As these files were 0 bytes, make distclean now
removes them.

We already patch checkpatch.pl, so we will just patch spdxcheck.py
when we add it and get rid of these directories.
They can be added back should the need arise.

Reported-by: Geordan Neukum <gneukum1@gmail.com>
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

LICENSES: adopt Linux-like LICENSES directory structure ...

At the moment
  grep -r --exclude-dir=.git 'SPDX-License-Identifier:' | \
    grep -v 'GPL-2.0'

shows me 39 non-dually-licensed source code files (SoCFPGA) that have a
BSD-3-Clause license _identifier_.
There seems to be no barebox BSD-3-Clause license  _text_ however,
which runs afoul of the first clause of the license.

To account for this and future contributions which are licensed under
non-GPL-2.0-but-compatible licenses, create a new LICENSES directory
like the one Linux has, where licenses are located.

This also removes the jumptable exception inherited from U-Boot:

  > On 11/2/19 09:00, Sascha Hauer wrote:
  >> I don't think we need this [exception]. barebox doesn't have
  >> (and never had since the fork from U-Boot) these standalone
  >> applications. There is no jumptable in barebox. What we do have
  >> is modules support, but these are not covered by this text.

The LICENSES/exceptions directory is therefore empty as well as
the LICENSES/other directory, which might house new licenses in future.
They are added along this commit however, because upstream
checkpatch.pl introduced with a later commit depends on their
existence.

Fixes: a3ffa97f4 ("rename U-Boot-v2 project to barebox")
Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>