Makefile: make $(BUILD_DIR)/.root rule idempotent

The $(BUILD_DIR)/.root rule is executed as part of the 'dirs'
target. The 'dirs' target is re-executed at every execution of 'make
external-deps', and make external-deps explicitly tells make to ignore
targets that have already been made (through the -B option). This
means that the $(BUILD_DIR)/.root rule has to be idempotant, which was
not the case this the introduction of the lib32/lib64 symbolic link.

Running 'make external-deps' three times in a row was sufficient to
trigger an error due to symbolic links being incorrectly created. This
patch fixes that.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

Makefile

@@ -451,9 +451,9 @@ $(BUILD_DIR)/.root:
 		--exclude .hg --exclude=CVS --exclude '*~' \
 		$(TARGET_SKELETON)/ $(TARGET_DIR)/
 	cp support/misc/target-dir-warning.txt $(TARGET_DIR_WARNING_FILE)
-	@ln -s lib $(TARGET_DIR)/$(LIB_SYMLINK)
+	@ln -snf lib $(TARGET_DIR)/$(LIB_SYMLINK)
 	@mkdir -p $(TARGET_DIR)/usr
-	@ln -s lib $(TARGET_DIR)/usr/$(LIB_SYMLINK)
+	@ln -snf lib $(TARGET_DIR)/usr/$(LIB_SYMLINK)
 	touch $@
 
 $(TARGET_DIR): $(BUILD_DIR)/.root