x86_64 demo: export GRUB_SERIAL_COMMAND and GRUB_CMDLINE_LINUX

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.

Signed-off-by: Curt Brune <curt@cumulusnetworks.com>