nova: uefi boot support using multiboot2

- kernel provides ACPI RSDT/XSDT pointer
- core exports it via a ROM called "platform_info"

Issue #2242

repos/base-nova/include/nova/syscall-generic.h

@@ -91,6 +91,8 @@ namespace Nova {
 		struct Mem_desc
 		{
 			enum Type {
+				ACPI_XSDT           = -4,
+				ACPI_RSDT           = -3,
 				MULTIBOOT_MODULE    = -2,
 				MICROHYPERVISOR     = -1,
 				AVAILABLE_MEMORY    =  1,