ndstool: Add support for game icons in GRF file format

[RyouArashi]

This patch adds support for game icons in GRF file format (converted using grit) to ndstool.

Usage:
Specify a .grf file instead of a .bmp file as the first parameter to the -b option.
The GRF file must contain a header, the tiled image data and a palette. This is
achieved by using the following command line:
grit <imagefile> -g -gt -gB4 -gT <color> -m! -p -pe 16 -fh! -ftr

Background:
Most graphics programs (such as MS Paint, Paint.net, etc.) simply
use palette entry 0 as the top-left pixel which is not necessarily
meant to be transparent.

Patch submitted under:
https://sourceforge.net/tracker/?func=d ... tid=668553