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 <[email protected]>
Signed-off-by: Thomas Petazzoni <[email protected]>
(cherry picked from commit ad980cc)
Signed-off-by: Peter Korsgaard <[email protected]>

Author: Trent Piepho

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"