petitboot: Implement ordered boot options

Move petitboot to a more familiar 'boot-order' based autoboot system.
The discover server now reads multiple values from the petitboot,bootdev
parameter and adds them in order to config->autoboot_opts. Boot priority
is determined by the options' position in the list.
On the client, nc-config now recognises the new boot order, and allows
the user to add, remove, and reorder the devices in the list.

Signed-off-by: Samuel Mendoza-Jonas <sam.mj@au1.ibm.com>