nrf/Makefile: Fix .PHONY target.
It must be in uppercase.
@@ -238,7 +238,7 @@ OBJ += $(BUILD)/pins_gen.o
$(BUILD)/$(FATFS_DIR)/ff.o: COPT += -Os
$(filter $(PY_BUILD)/../extmod/vfs_fat_%.o, $(PY_O)): COPT += -Os
-.phony: all flash sd binary hex
+.PHONY: all flash sd binary hex
all: binary hex
