libiio: new package

Libiio is a library to ease the development of software interfacing
Linux Industrial I/O (IIO) devices.

http://wiki.analog.com/resources/tools-software/linux-software/libiio



[Thomas:
 - Remove changelog from the commit log.
 - Rename init script to S99iiod instead of S99iiod.sh
 - Remove "status" command from init script, and implement "restart"
   instead.
 - Add dependency on thread support in toolchain.
 - Fixup indentation in the .mk file.
 - Fixup wrapping in the Config.in help text.]

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>