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>