Fix buildroot not to modify Rules.mak. Patch From vapier.

toolchain/uClibc/uclibc.mk

@@ -51,8 +51,8 @@ $(UCLIBC_DIR)/.unpacked: $(DL_DIR)/$(UCLIBC_SOURCE)
 	touch $(UCLIBC_DIR)/.unpacked
 
 $(UCLIBC_DIR)/.configured: $(UCLIBC_DIR)/.unpacked
-	$(SED) 's,^CROSS=.*,CROSS=$(TARGET_CROSS),g' $(UCLIBC_DIR)/Rules.mak
 	cp $(UCLIBC_CONFIG_FILE) $(UCLIBC_DIR)/.config
+	$(SED) 's,^CROSS_COMPILER_PREFIX=.*,CROSS_COMPILER_PREFIX="$(TARGET_CROSS)",g' $(UCLIBC_DIR)/.config \
 	$(SED) 's,^.*TARGET_$(UCLIBC_TARGET_ARCH).*,TARGET_$(UCLIBC_TARGET_ARCH)=y,g' \
 		$(UCLIBC_DIR)/.config
 	$(SED) 's,^TARGET_ARCH.*,TARGET_ARCH=\"$(UCLIBC_TARGET_ARCH)\",g' $(UCLIBC_DIR)/.config