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
diff --git a/init/init.h b/init/init.h
index cfb3139..4e4da32 100644
--- a/init/init.h
+++ b/init/init.h
@@ -36,4 +36,6 @@
 
 int add_environment(const char* key, const char* val);
 
+bool wait_property(const char *name, const char *value);
+
 #endif  /* _INIT_INIT_H */