Provide Weston IVI-Shell, run it on demand

Weston IVI-Shell provides a shell plugin for Weston,
mapping the GENIVI API (http://www.genivi.org) for
In-Vehicle Infotainment.

This is a code backport from Weston 1.9.0 "release".

This patch makes sure that Weston IVI-Shell always gets
built ; it will not be started, however, unless the builder
specifies the following in his "conf/local.conf" file :

IMAGE_INSTALL_append = " \
    weston-ivi-shell-config \
    "

or manually overwrites the "/etc/xdg/weston/weston.ini"
file with a correct one.

Change-Id: I033094aaf8ac2c6299bd354b362b24ffc9ed8577
Signed-off-by: Manuel Bachmann <manuel.bachmann@iot.bzh>