endeavoru/enrc2b: Workaround disallowed 'start gsm0710mux' command

init complained about

    init: sys_prop: Unable to start service ctl [gsm0710mux] uid:0 gid:1001 pid:642

Work around that by setting a ril property and handling the start in the
init.tegra3.rc script.

Change-Id: I8b2c7d67de98784993401eef48254bf753ae3e7c