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.
Rename ACPI initialize function.

  * i386/i386at/acpi/boot.c(acpi_table_init): Rename.
  * i386/i386at/acpi/model_dep.c(c_boot_entry): Likewise.
  * include/mach/acpi/table.h(acpi_table_init): Likewise.
Make table checksum function as generic.

  * i386/i386at/acpi/boot.c(checksum): Make as generic.