weston: Launch even without input devices

Fix the recurring bug and launch Weston 2.0 even
if there are no input devices such a keyboard
or a mouse.

Bug-AGL: SPEC-936

Change-Id: I9c92ce7762626fd5df40224b53317c1239a4bd6f
Signed-off-by: Leon Anavi <leon.anavi@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/11203
Reviewed-by: Thomas Rini <trini@konsulko.com>
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Changhyeok Bae <changhyeok.bae@gmail.com>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>