kconfig: add support for warnings when enabling deprecated features

This adds two new Kconfig settings.

The first setting `DEPRECATED` which is a promptless symbol.
This symbol must be selected by any deprecated setting when enabled.

The second setting is `WARN_DEPRECATED` which is a user controlled
setting that configures the build system to print a warning when a
deprecated feature is enabled.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>

Kconfig.zephyr

@@ -600,6 +600,20 @@ config BUILD_OUTPUT_META_STATE_PROPAGATE
 
 endmenu
 
+config DEPRECATED
+	bool
+	help
+	  Symbol that must be selected by a feature or module if it is
+	  considered to be deprecated.
+
+config WARN_DEPRECATED
+	bool
+	default y
+	prompt "Warn on deprecated usage"
+	help
+	  Print a warning when the Kconfig tree is parsed if any deprecated
+	  features are enabled.
+
 config EXPERIMENTAL
 	bool
 	help

scripts/kconfig/kconfig.py

@@ -62,6 +62,9 @@ def main():
         check_assigned_sym_values(kconf)
         check_assigned_choice_values(kconf)
 
+    if kconf.syms['WARN_DEPRECATED'].tri_value == 2:
+        check_deprecated(kconf)
+
     if kconf.syms['WARN_EXPERIMENTAL'].tri_value == 2:
         check_experimental(kconf)
 
@@ -205,6 +208,17 @@ Practices sections of the manual might be helpful too.\
 """
 
 
+def check_deprecated(kconf):
+    deprecated = kconf.syms['DEPRECATED']
+    dep_expr = deprecated.rev_dep
+
+    if dep_expr is not kconf.n:
+        selectors = [s for s in split_expr(dep_expr, OR) if expr_value(s) == 2]
+        for selector in selectors:
+            selector_name = split_expr(selector, AND)[0].name
+            warn(f'Deprecated symbol {selector_name} is enabled.')
+
+
 def check_experimental(kconf):
     experimental = kconf.syms['EXPERIMENTAL']
     dep_expr = experimental.rev_dep