x86_64: demo installer may destruct ONIE's serial configs

After the commit f0901d4, demo installer does not export
'GRUB_SERIAL_COMMAND' and 'GRUB_CMDLINE_LINUX' before calling
'50_onie_grub'. It always writes default serial configs into
grub.cfg in onie-boot. ONIE's serial console will be out of
control after installing the demo os.