kconfig: document use of CONFIG_ environment variable

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

Documentation/kbuild/kconfig.txt

@@ -46,6 +46,12 @@ KCONFIG_OVERWRITECONFIG
 If you set KCONFIG_OVERWRITECONFIG in the environment, Kconfig will not
 break symlinks when .config is a symlink to somewhere else.
 
+CONFIG_
+--------------------------------------------------
+If you set CONFIG_ in the environment, Kconfig will prefix all symbols
+with its value when saving the configuration, instead of using the default,
+"CONFIG_".
+
 ______________________________________________________________________
 Environment variables for '{allyes/allmod/allno/rand}config'