hw/i386/fw_cfg: Check ACPI availability with acpi_builtin()

Define acpi_tables / acpi_tables_len stubs, then replace the
compile-time CONFIG_ACPI check in fw_cfg.c by a runtime one.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Ani Sinha <anisinha@redhat.com>
Message-Id: <20250307223949.54040-4-philmd@linaro.org>

hw/acpi/acpi-stub.c

@@ -21,6 +21,9 @@
 #include "qemu/osdep.h"
 #include "hw/acpi/acpi.h"
 
+char unsigned *acpi_tables;
+size_t acpi_tables_len;
+
 void acpi_table_add(const QemuOpts *opts, Error **errp)
 {
     g_assert_not_reached();

hw/i386/fw_cfg.c

@@ -145,10 +145,10 @@ FWCfgState *fw_cfg_arch_create(MachineState *ms,
      */
     fw_cfg_add_i16(fw_cfg, FW_CFG_MAX_CPUS, apic_id_limit);
     fw_cfg_add_i64(fw_cfg, FW_CFG_RAM_SIZE, ms->ram_size);
-#ifdef CONFIG_ACPI
-    fw_cfg_add_bytes(fw_cfg, FW_CFG_ACPI_TABLES,
-                     acpi_tables, acpi_tables_len);
-#endif
+    if (acpi_builtin()) {
+        fw_cfg_add_bytes(fw_cfg, FW_CFG_ACPI_TABLES,
+                         acpi_tables, acpi_tables_len);
+    }
     fw_cfg_add_i32(fw_cfg, FW_CFG_IRQ0_OVERRIDE, 1);
 
     fw_cfg_add_bytes(fw_cfg, FW_CFG_HPET, &hpet_fw_cfg, sizeof(hpet_fw_cfg));