tools: mtk_image: split the code of generating NAND header into a new file

The predefined NAND headers take too much spaces in the mtk_image.c.
Moving them into a new file can significantly improve the readability of
both mtk_image.c and the new mtk_nand_headers.c.

This is a preparation for adding more NAND headers.

Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Daniel Golle <daniel@makrotopia.org>
diff --git a/tools/Makefile b/tools/Makefile
index 3626919..34a1aa7 100644
--- a/tools/Makefile
+++ b/tools/Makefile
@@ -147,6 +147,7 @@
 			gpimage.o \
 			gpimage-common.o \
 			mtk_image.o \
+			mtk_nand_headers.o \
 			$(ECDSA_OBJS-y) \
 			$(RSA_OBJS-y) \
 			$(AES_OBJS-y)