[acpi] Expose ACPI tables via settings mechanism

Allow values to be read from ACPI tables using the syntax

  ${acpi/<signature>.<index>.0.<offset>.<length>}

where <signature> is the ACPI table signature as a 32-bit hexadecimal
number (e.g. 0x41504093 for the 'APIC' signature on the MADT), <index>
is the index into the array of tables matching this signature,
<offset> is the byte offset within the table, and <length> is the
field length in bytes.

Numeric values are returned in reverse byte order, since ACPI numeric
values are usually little-endian.

For example:

  ${acpi/0x41504943.0.0.0.0}           - entire MADT table in raw hex
  ${acpi/0x41504943.0.0.0x0a.6:string} - MADT table OEM ID
  ${acpi/0x41504943.0.0.0x24.4:uint32} - local APIC address

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/config/config.c

@@ -337,6 +337,9 @@ REQUIRE_OBJECT ( memmap_settings );
 #ifdef VRAM_SETTINGS
 REQUIRE_OBJECT ( vram_settings );
 #endif
+#ifdef ACPI_SETTINGS
+REQUIRE_OBJECT ( acpi_settings );
+#endif
 
 /*
  * Drag in selected keyboard map

src/config/settings.h

@@ -14,6 +14,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 //#define	MEMMAP_SETTINGS	/* Memory map settings */
 //#define	VMWARE_SETTINGS	/* VMware GuestInfo settings */
 //#define	VRAM_SETTINGS	/* Video RAM dump settings */
+//#define	ACPI_SETTINGS	/* ACPI settings */
 
 #include <config/named.h>
 #include NAMED_CONFIG(settings.h)