Add support to pass boothowto and bootdev info from bootloader to kernel.

Kernel side changes:
- use %d7 (boothowto) and %d6 (bootdev) to pass info and also use
  old macro in <sys/reboot.h> as ancient 4.4BSD did for simplicity
- add <machine/bootinfo.h> to define values as API to pass these info
- save boothowto and bootdev in registers right after zero'ing bss
- add MD device_register(9) to check booted_device per passed bootdev info
- merge old bootarg checks in rootconf() and luna68k_init() with
  tweaks for backward compatibility
  (direct boot a.out kernel from ROM monitor without bootloader still works)

(tsutsui)