Use boot scr

[Percentnineteen]

This PR is a better version of the previous one I opened and closed. It does a few things to support all devices of the family equally:

• move board detection logic to board.c
• remove fdt modification
• enable the poweroff command in u-boot menu (mostly a testing QOL thing I left in)
• boot logic changed to boot:
  • boot.scr on internal if it exists
  • boot.scr on sd card if it exists
  • fall back to grub if neither
• fixed the volume up button working in u-boot/grub menus
• fixed the display for the mini devices
• silenced a bunch of stuff to keep the boot clean