[packages] asterisk-11.x: add new package

LTS version of Asterisk has been recently released. This package is a
bleeding-edge alternative to existing asterisk-1.8.x package.

Signed-off-by: Jiri Slachta <slachta@cesnet.cz>
Signed-off-by: Daniel Golle <dgolle@allnet.de>

git-svn-id: svn://svn.openwrt.org/openwrt/packages@35540 3c298f89-4303-0410-b956-a3cf2f4a3e73