efi: Move EFI_FDT_USE_INTERNAL to a common header

Move this constant to the efi.h header so that it can be used by the
app.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/efi.h

@@ -566,6 +566,9 @@ extern char _binary_u_boot_bin_start[], _binary_u_boot_bin_end[], _binary_u_boot
 				EFI_VARIABLE_APPEND_WRITE | \
 				EFI_VARIABLE_ENHANCED_AUTHENTICATED_ACCESS)
 
+/* Use internal device tree when starting UEFI application */
+#define EFI_FDT_USE_INTERNAL NULL
+
 /**
  * efi_get_priv() - Get access to the EFI-private information
  *