Config.in: Document BR2_CCACHE_DIR override

This variable, like BR2_DL_DIR, is designed to be overridable from the
environment.  Unlike BR2_DL_DIR, it is not documented as such in the
Config.in help text.  Do so now.

Signed-off-by: Trent Piepho <tpiepho@impinj.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit ad980ccc3639baa2e517c4d36e836b71ab9f5b8f)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

Config.in

@@ -332,6 +332,9 @@ config BR2_CCACHE_DIR
 	default "$(HOME)/.buildroot-ccache"
 	help
 	  Where ccache should store cached files.
+	  If the Linux shell environment has defined the BR2_CCACHE_DIR
+	  environment variable, then this overrides this configuration
+	  item.
 
 config BR2_CCACHE_INITIAL_SETUP
 	string "Compiler cache initial setup"