Kconfig: flag LEGACY_INCLUDE_PATH as deprecated

Deprecate the LEGACY_INCLUDE_PATH so that it can be removed down the
road.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>
diff --git a/Kconfig.zephyr b/Kconfig.zephyr
index 0bdf18d..7a3b2d6 100644
--- a/Kconfig.zephyr
+++ b/Kconfig.zephyr
@@ -844,12 +844,11 @@
 endmenu
 
 config LEGACY_INCLUDE_PATH
-	bool "Allow for the legacy include paths (without the zephyr/ prefix)"
+	bool "Allow for the legacy include paths (without the zephyr/ prefix) (DEPRECATED)"
 	help
-	  Allow applications and libraries to use the Zephyr legacy include
-	  path which does not use the zephyr/ prefix. For example, the
+	  DEPRECATED: Allow applications and libraries to use the Zephyr legacy
+	  include path which does not use the zephyr/ prefix. For example, the
 	  preferred way to include a Zephyr header is to use <zephyr/kernel.h>,
-	  but enabling CONFIG_LEGACY_INCLUDE_PATH will allow developers to
-	  use <kernel.h> instead. This (without the zephyr/ prefix) is
-	  deprecated and should be avoided. Eventually, it will not be
-	  supported.
+	  but enabling CONFIG_LEGACY_INCLUDE_PATH will allow developers to use
+	  <kernel.h> instead. This (without the zephyr/ prefix) is deprecated
+	  and should be avoided. Eventually, it will not be supported.