init: add wait_for_prop builtin command

There are many use cases from vendors to exec service in background and then
use a shell scriprt to wait for the command done.

This CL is to add a wait_for_prop command to suppor those use cases.

Bug: 34746108
Test: on marlin
Change-Id: Ia81290b0928f9d375710d2daa546714f0cd65b72