blackbox: use BR2_SOURCEFORGE_MIRROR

Don't hardcode specific sourceforge mirrors.

package/blackbox/blackbox.mk

@@ -6,7 +6,7 @@
 
 BLACKBOX_VERSION = 0.70.1
 BLACKBOX_SOURCE = blackbox-$(BLACKBOX_VERSION).tar.bz2
-BLACKBOX_SITE = http://superb-west.dl.sourceforge.net/sourceforge/blackboxwm/
+BLACKBOX_SITE = http://$(BR2_SOURCEFORGE_MIRROR).dl.sourceforge.net/sourceforge/blackboxwm/
 BLACKBOX_AUTORECONF = NO
 BLACKBOX_INSTALL_STAGING = NO
 BLACKBOX_INSTALL_TARGET = YES