efi: Adjust script to show pre-relocation output on terminal

When running with video enabled, the pre-relocation output of U-Boot is
currently lost. Add a -serial flag to show it on the terminal.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/scripts/build-efi.sh b/scripts/build-efi.sh
index bc9aeeb..46c2880 100755
--- a/scripts/build-efi.sh
+++ b/scripts/build-efi.sh
@@ -96,6 +96,8 @@
 	fi
 	if [[ -n "${serial}" ]]; then
 		extra="-display none -serial mon:stdio"
+	else
+		extra="-serial mon:stdio"
 	fi
 	echo "Running ${qemu}"
 	# Use 512MB since U-Boot EFI likes to have 256MB to play with