Update grub configuration.

* Add scripts/create-grub-dir.sh to build templates/boot/grub/
* Add font file templates/boot/grub/ascii.pf2
* Update templates/boot/grub/ based on grub-pc_1.98-1_i386.deb
* grub.cfg:
  - add additional menu_color_* entries if loading background image
  - loadfont /boot/grub/ascii.pf2
  - increase timeout to 20 seconds
  - replace terminal command with terminal_output (sucking non-backward compability!)
  - use gfxpayload=1024x768 instead of vga=...