Detect ACPI RSDP/RSDT

  * i386/Makefrag.am: Add acpi sources.
  * i386/configfrag.ac: Add acpi configure flag.
  * i386/i386at/acpi.c: New source for ACPI support.
  * i386/i386at/acpi.h: Likewise.
  * i386/i386at/model_dep.c (c_boot_entry): Call ACPI init function.