[fdt] Add the "fdt" command

Allow a Flattened Device Tree blob (DTB) to be provided to a booted
operating system using a script such as:

  #!ipxe
  kernel /images/vmlinuz console=ttyAMA0
  initrd /images/initrd.img
  fdt /images/rk3566-radxa-zero-3e.dtb
  boot

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

src/config/defaults/efi.h

@@ -64,10 +64,12 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
 #if defined ( __arm__ ) || defined ( __aarch64__ )
 #define IOAPI_ARM
+#define FDT_CMD
 #endif
 
 #if defined ( __aarch64__ )
 #define	IMAGE_GZIP		/* GZIP image support */
+#define FDT_CMD
 #endif
 
 #if defined ( __loongarch__ )
@@ -76,6 +78,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
 #if defined ( __riscv )
 #define IOAPI_RISCV
+#define FDT_CMD
 #endif
 
 #endif /* CONFIG_DEFAULTS_EFI_H */

src/config/defaults/sbi.h

@@ -32,5 +32,6 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
 #define REBOOT_CMD
 #define POWEROFF_CMD
+#define FDT_CMD
 
 #endif /* CONFIG_DEFAULTS_SBI_H */