Add some waits in Ubuntu boot_command
Add some waits in Ubuntu boot_command

The vmlinuz and initrd commands do take some time to execute so
we need to add some sleeps here to ensure it boots correctly

Change-Id: If43608c401d2bfab256e1ac485a353e40a9386aa

NeCTAR-RC/nectar-images,NeCTAR-RC/nectar-images