Refactor.

  * Makefrag.am: Add rule for acpi.
  * configfrag.ac: Likewise.
  * i386/Makefrag.am: Rename target file.
  * i386/configfrag.ac: Revert.
  * i386/i386at/acpi.c: Rename to i386/i386at/acpi/boot.c.
  * i386/i386at/acpi.h: Rename to include/mach/acpi/table.h.