color framebuffer support for luna68k

Hi,

I've committed preleminary support for LUNA's 4bpp color framebuffer:
 http://mail-index.netbsd.org/source-changes/2013/12/28/msg050266.html

With this new driver and recently updated mlterm-fb 3.3.2 in pkgsrc,
now we can demonstrate "Twitter timeline with color icons and wallpaper"
on old OMRON LUNA, with 20MHz MC68030 and 16MB RAM ;-)
 http://www.youtube.com/watch?v=4igLujPyK0M

Special thanks to Araki Ken, for adding 4bpp framebuffer support
to the latest mlterm-3.3.2:
 http://mail-index.netbsd.org/pkgsrc-changes/2013/12/25/msg099098.html
and also preparing modified rubygem "tw" client that supports
icons and pictures using sixel graphics for mlterm:
 https://bitbucket.org/arakiken/tw/src

Have fun,
---
Izumi Tsutsui