Add userspace_reboot.mk and include it in mainline_system.mk

The intention of userspace_reboot.mk is to provide a default
configuration (e.g. userspace reboot watchdog timeout) for devices that
are supporting userspace reboot feature. So far it only contains one
property that tells whenever device supports userspace reboot, more to
follow in separate CLs.

Test: flashed blueline, checked that property is set to true
Bug: 135984674
Change-Id: I32e6281ccc36236ec4fa5cde299753b6e0ece181