apps/nsh_commands: Use quit to poweroff

Currently new users when try to run NuttX using the SIM
get stuck into simulator, because they have no idea that
poweroff command is used to leave it. Let use KISS approach
and use quit as an alias to poweroff command.

Signed-off-by: Alan C. Assis <acassis@gmail.com>
diff --git a/nshlib/nsh_command.c b/nshlib/nsh_command.c
index 6f23f73..a505b1f 100644
--- a/nshlib/nsh_command.c
+++ b/nshlib/nsh_command.c
@@ -456,6 +456,7 @@
 
 #if defined(CONFIG_BOARDCTL_POWEROFF) && !defined(CONFIG_NSH_DISABLE_POWEROFF)
   CMD_MAP("poweroff", cmd_poweroff, 1, 2, NULL),
+  CMD_MAP("quit", cmd_poweroff, 1, 2, NULL),
 #endif
 
 #ifndef CONFIG_NSH_DISABLE_PRINTF