init: Implement 'exec' command.

(cherry-pick of d05ab3952ec0e38f33a0e80ce6b9eb45b0064ba4.)

Change-Id: Id6d9bb32e51a0ad090ed8240cc505dc45b57b35d
diff --git a/init/init_parser.c b/init/init_parser.c
index a124fa2..02cbf3d 100644
--- a/init/init_parser.c
+++ b/init/init_parser.c
@@ -97,6 +97,7 @@
     case 'e':
         if (!strcmp(s, "nable")) return K_enable;
         if (!strcmp(s, "xec")) return K_exec;
+        if (!strcmp(s, "xeconce")) return K_execonce;
         if (!strcmp(s, "xport")) return K_export;
         break;
     case 'g':