New kernel modes (including 800x600@56Hz !)

Since a recent commit, developers now have a better way to set modes.

The kconf.h kernel configuration file allows now several modes, selectable through your project Makefile config options.

Current defined timings include  :

- 640x480, 60Hz @ 168MHz clock (default one), 7 clocks per pixel
- 320x480, 60Hz @ 168MHz clock (repeat each line twice to use as 320x240 display)
- 640x480, 60Hz, overclocked at 192 MHZ,  7 clocks per pixel (longer vsync time)
- 800x600, 56Hz ! overclocked a bit at 180MHz. Yes the bitbox can do 800x600, 56fps.
You only have 5 clocks per pixels here however, so you'd better optimize that blit loop.

other modes can be created by adding config lines to the kconf.h file, all timing computation is based on number of pixels for Porch begin/end/sync and display sizes in pixels ; however be sure to get the right clock ratios to run USB/SDIO clocks at 48MHz !

yes, testkernel can work at 800x600 (sorry bad pic)