scripts: build-efi: Flush output before running QEMU

Complete writing of any output before running QEMU, since otherwise it
does not always appear until QEMU exits (e.g. with 'tee')

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

scripts/build-efi

@@ -19,6 +19,7 @@ from argparse import ArgumentParser
 import os
 from pathlib import Path
 import shutil
+import sys
 
 import build_helper
 
@@ -152,6 +153,7 @@ class BuildEfi:
         cmd += extra
         self.helper.add_qemu_args(self.args, cmd, base_hd=1)
         tout.info(' '.join(cmd))
+        sys.stdout.flush()
         command.run(*cmd)
 
     def setup_files(self, build, build_type, dst):