buildroot

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>