Add xwayland path setting.

This makes it possible to use SOMMELIER_XWAYLAND_PATH and
--xwayland-path=PATH to specific the full path to the Xwayland
binary.

Bug: 826923
Change-Id: I0156f18976a3c4e43a70de95b49eca398368cad6
Reviewed-on: https://chromium-review.googlesource.com/997995
Tested-by: David Reveman <reveman@chromium.org>
Reviewed-by: Stephen Barber <smbarber@chromium.org>
1 file changed