BugLink: http://bugs.launchpad.net/bugs/1700972

Allow images to be created without the need of an initrd and also allow
users to run without an initrd if they want to.
Signed-off-by: Marcelo Henrique Cerri <marcelo.cerri@canonical.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>