Convert CONFIG_SYS_MAXARGS to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_MAXARGS

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 09193b6..dd43358 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -71,6 +71,10 @@
 	  printed when the command interpreter needs more input
 	  to complete a command. Usually "> ".
 
+config SYS_MAXARGS
+	int "Maximum number arguments accepted by commands"
+	default 16
+
 config SYS_XTRACE
 	bool "Command execution tracer"
 	depends on CMDLINE