libunistring: new package

[Thomas:
  - adjust the title of the commit
  - remove the thread dependency, by using the --disable-threads
    option when needed
  - remove LIBUNISTRING_SOURCE, since its value was the default
  - use BR2_GNU_MIRROR in LIBUNISTRING_SITE
  - adjust the license, which really is LGPLv3+, not LGPLv2. The
    license file is COPYING.LIB.]

Signed-off-by: Pedro Aguilar <paguilar@paguilar.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>