efi_loader: error message if BootOrder not defined

For booting via `bootefi bootmgr` it is necessary that the EFI variable
BootOrder is defined. Provide a diagnostic message if the variable is
missing.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/lib/efi_loader/efi_bootmgr.c b/lib/efi_loader/efi_bootmgr.c
index 196116b..f4b1889 100644
--- a/lib/efi_loader/efi_bootmgr.c
+++ b/lib/efi_loader/efi_bootmgr.c
@@ -184,8 +184,10 @@
 	rs = systab.runtime;
 
 	bootorder = get_var(L"BootOrder", &efi_global_variable_guid, &size);
-	if (!bootorder)
+	if (!bootorder) {
+		printf("BootOrder not defined\n");
 		goto error;
+	}
 
 	num = size / sizeof(uint16_t);
 	for (i = 0; i < num; i++) {