libxx: rename NUTTX_LIBCXX

NUTTX_LIBCXX is only basic C++ support, do not support many c++
library function, NUTTX_NOCXXLIB is more suitable.
When NUTTX_NOCXXLIB is select, do not select lower level support.

Change-Id: I158937a4ba02f8fa3c76101acbdfb295d8cd736f

libs/libxx/Kconfig

@@ -24,14 +24,14 @@ if HAVE_CXX
 
 choice
 	prompt "C++ Library"
-	default NUTTX_LIBXX
+	default NOCXXLIB
 
-config NUTTX_LIBXX
-	bool "NuttX C++ library"
+config NOCXXLIB
+	bool "NuttX Basic C++ support"
 	---help---
 		A fragmentary C++ library that will allow to build only
-		the simplest of C++ applications.  In the deeply embedded world,
-		that is probably all that is necessary.
+		the simplest of C++ applications. Only contain basic C++
+		runtime support function.
 
 config LIBCXX
 	bool "LLVM libc++ C++ Standard Library"
@@ -50,6 +50,8 @@ config UCLIBCXX
 
 endchoice
 
+if LIBCXX || UCLIBCXX
+
 choice
 	prompt "C++ low level library select"
 	default CXX_LIBSUPCXX
@@ -69,6 +71,8 @@ config CXX_LIBSUPCXX
 
 endchoice
 
+endif
+
 config HAVE_CXXINITIALIZE
 	bool "Have C++ initialization"
 	default LIBCXX || UCLIBCXX