ivi-shell: use install paths in example config

The example weston.ini file uses source and build
directory paths. Therefore, it is only useful when
used on the same system that is used to build Weston.

We can use install paths instead of build/source paths
to fix this problem.

v2 changes:
- use $(westondatadir) instead of $(datadir)

Reported-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Emre Ucan <eucan@de.adit-jv.com>
Reviewed-by: Michael Tretter <m.tretter@pengutronix.de>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>