shairplay-sync: new package

[Thomas:
 - rework init script to be more like other Buildroot init scripts,
   and fix indentation.
 - fix indentation in the .mk file
 - remove dependencies on host-autoconf and host-libtool, since the
   package has AUTORECONF = YES, this is not needed.
 - fix licensing informations.]

Signed-off-by: Jörg Krause <jkrause@posteo.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>