uptime command, unconditional because it's very small

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>

shell/cmd_uptime.c

+#include <wrc.h>
+#include "shell.h"
+
+static int cmd_uptime(const char *args[])
+{
+	extern uint32_t uptime_sec;
+
+	pp_printf("%u\n", uptime_sec);
+	return 0;
+}
+
+DEFINE_WRC_COMMAND(uptime) = {
+	.name = "uptime",
+	.exec = cmd_uptime,
+};

shell/shell.mk

@@ -5,6 +5,7 @@ obj-$(CONFIG_WR_NODE) += \
 	shell/cmd_help.o \
 	shell/cmd_mac.o \
 	shell/cmd_ps.o \
+	shell/cmd_uptime.o \
 	shell/cmd_refresh.o
 
 obj-$(CONFIG_EMBEDDED_NODE) += \