imx: Drop CONFIG_USE_PLUGIN

This option is not defined anywhere. Drop it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/freescale/mx7ulp_evk/Makefile b/board/freescale/mx7ulp_evk/Makefile
index 686763d..9f33c61 100644
--- a/board/freescale/mx7ulp_evk/Makefile
+++ b/board/freescale/mx7ulp_evk/Makefile
@@ -2,7 +2,3 @@
 # (C) Copyright 2016 Freescale Semiconductor, Inc.
 
 obj-y  := mx7ulp_evk.o
-
-extra-$(CONFIG_USE_PLUGIN) :=  plugin.bin
-$(obj)/plugin.bin: $(obj)/plugin.o
-	$(OBJCOPY) -O binary --gap-fill 0xff $< $@