Fixes incorrect makefile syntax

fixes #379

Author: kdschlosser

ext_mod/lvgl/micropython.mk

@@ -15,7 +15,7 @@ CFLAGS_USERMOD += -I$(LVGL_DIR)
 CFLAGS_USERMOD += -I$(LIB_DIR)
 
 
-ifdef $(LV_CFLAGS)
+ifdef LV_CFLAGS
     CFLAGS_USERMOD += $(LV_CFLAGS)
 endif