xbps-src: added suport for XBPS_CACHEDIR in configuration file.

XBPS_CACHEDIR will be bind mounted as read/write in target masterdir
into /cachedir. This directory must have read/write perms for the user
running xbps-src.

xbps-src/shutils/init_funcs.sh.in

@@ -65,6 +65,9 @@ set_defvars()
 	if [ -n "$xver" ]; then
 		# XBPS utils >= 0.9.0.
 		xbps_conf="-C $XBPS_MASTERDIR/usr/local/etc/xbps-conf.plist"
+		if [ -n "$XBPS_CACHEDIR" ]; then
+			xbps_conf="$xbps_conf -c $XBPS_CACHEDIR"
+		fi
 	fi
 	export XBPS_PKGDB_CMD="xbps-uhelper.static -r $XBPS_MASTERDIR"
 	export XBPS_BIN_CMD="xbps-bin.static $xbps_conf -r $XBPS_MASTERDIR"