Be extra careful about where the uClibc config file comes from

toolchain/uClibc/uclibc.mk

@@ -4,11 +4,13 @@
 #
 #############################################################
 
+ifndef $(UCLIBC_CONFIG_FILE)
 ifeq ($(BR2_ENABLE_LOCALE),y)
 UCLIBC_CONFIG_FILE=toolchain/uClibc/uClibc.config-locale
 else
 UCLIBC_CONFIG_FILE=toolchain/uClibc/uClibc.config
 endif
+endif
 
 ifeq ($(BR2_UCLIBC_VERSION_SNAPSHOT),y)
 # Be aware that this changes daily....