lib: cpp: Rename CONFIG_CPLUSPLUS to CONFIG_CPP

There is no reason for the top-level C++ Kconfig symbol to be called
`CPLUSPLUS` when the convention used throughout the Zephyr codebase is
to use "cpp" to refer to C++ and the rest of the C++-related Kconfig
symbols are prefixed with `CPP_`.

This commit renames the Kconfig `CPLUSPLUS` symbol to `CPP` in order to
better align with the de-facto standard naming convention used throughout
the Zephyr codebase, and deprecates the old `CPLUSPLUS` symbol.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>

lib/cpp/Kconfig

@@ -3,12 +3,14 @@
 # Copyright (c) 2018 B. Leforestier
 # SPDX-License-Identifier: Apache-2.0
 
-menuconfig CPLUSPLUS
+menu "C++ Language Support"
+
+config CPP
 	bool "C++ support for the application"
 	help
 	  This option enables the use of applications built with C++.
 
-if CPLUSPLUS
+if CPP
 
 choice
 	prompt "C++ Standard"
@@ -94,4 +96,8 @@ config CPP_STATIC_INIT_GNU
 	help
 	  GNU-compatible initialization of CPP static objects
 
-endif # CPLUSPLUS
+endif # CPP
+
+rsource "Kconfig.deprecated"
+
+endmenu

lib/cpp/Kconfig.deprecated

@@ -0,0 +1,15 @@
+# Copyright (c) 2022 Nordic Semiconductor ASA
+# SPDX-License-Identifier: Apache-2.0
+
+menu "Deprecated"
+
+config CPLUSPLUS
+	bool "[DEPRECATED] C++ support for the application"
+	select CPP
+	select DEPRECATED
+	help
+	  This option enables the use of applications built with C++.
+
+	  [DEPRECATED] Select CPP instead.
+
+endmenu