xxhash: new package

"xxHash is an extremely fast hash algorithm, running at RAM speed
limits."

[Thomas:
 - tweak commit log title
 - add correct license and license files information, using the
   suggestion from Yann E. Morin
 - add $(TARGET_MAKE_ENV) in the environment in the build step.]

Signed-off-by: Ed Swierk <eswierk@skyportsystems.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>