Fix EFI signing

buildbot/buildbot_steps.sh

@@ -206,6 +206,8 @@ function make_ipxe_efi() {
     embed_file="$(realpath "${embed_file}")"
     signing_cert="$(realpath "${signing_cert}")"
     ca_cert="$(realpath "${ca_cert}")"
+    efi_key="$(realpath "${efi_key}")"
+    efi_cert="$(realpath "${efi_cert}")"
 
     if [ ! -d "$ipxe_src_dir" ]; then
         >&2 echo "IPXE supplied directory does not exist."