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>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>