nsh:support wait command to wait task exit.

usage like:
  nsh> sleep 10 &
  [5:100]
  nsh>set pid1=$!
  nsh>sleep 15 &
  [6:100]
  nsh>set pid2=$!
  nsh>wait $pid1 $pid2

'wait' command will block nsh from running until pid1 and pid2 finish.
This is useful for parallel requirements to perform certain tasks

Signed-off-by: anjiahao <anjiahao@xiaomi.com>

nshlib/nsh_command.c

@@ -666,6 +666,11 @@ static const struct cmdmap_s g_cmdmap[] =
 
 #ifndef CONFIG_NSH_DISABLE_XD
   CMD_MAP("xd",       cmd_xd,       3, 3, "<hex-address> <byte-count>"),
+#endif
+#if !defined(CONFIG_NSH_DISABLE_WAIT) && defined(CONFIG_SCHED_WAITPID) && \
+    !defined(CONFIG_DISABLE_PTHREAD)
+  CMD_MAP("wait",     cmd_wait,     1, CONFIG_NSH_MAXARGUMENTS,
+          "pid1 [pid2 [pid3] ...]"),
 #endif
   CMD_MAP(NULL,       NULL,         1, 1, NULL)
 };