change a boot logo in linux kernel

* Drawing a new boot logo by image tool such a GIMP.

* Install "ppmquant" by

# sudo apt-get install netpbm

* Make the image with the depth of colors is below 224

# ppmquant 224 "logofile" > "newlogofile"

* Make the image with plain text

# pnmnoraw "newlogofile" > logo_linux_clut224.ppm

* copy logo_linux_clut224.ppm to "drivers/video/logo"