set TOOLCHAIN_DIR var when using external toolchain

This patch allows to use the openwrt toolchain as external toolchain for the
same project. See bug #9131 for details.

Signed-off-by: Sven Bachmann <dev@mcbachmann.de>

SVN-Revision: 26405

rules.mk

@@ -140,6 +140,7 @@ ifndef DUMP
       ifneq ($(TOOLCHAIN_LIB_DIRS),)
         TARGET_LDFLAGS+= $(patsubst %,-L%,$(TOOLCHAIN_LIB_DIRS))
       endif
+      TOOLCHAIN_DIR:=$(TOOLCHAIN_ROOT_DIR)
     endif
   endif
 endif