[memmap] Define an API for managing the system memory map

Define a generic system memory map API, based on the abstraction
created for parsing the FDT memory map and adding a concept of hidden
in-use memory regions as required to support patching the BIOS INT 15
memory map.

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

src/config/defaults/efi.h

@@ -16,6 +16,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #define CONSOLE_EFI
 #define TIMER_EFI
 #define UMALLOC_EFI
+#define MEMMAP_NULL
 #define SMBIOS_EFI
 #define SANBOOT_EFI
 #define BOFM_EFI