discover: Integrate ipmi bootdev settings into the priority system

discover: Integrate ipmi bootdev settings into the priority system

Currently, we expose the boot device priorities through an array in
struct config, which will either be the default (network -> disk), or a
single device type specified by the IPMI code.

Rather than hide the implementation details in this array, we'd like to
expose the details of the machine configuration instead. This allows
user visibility of the real boot configuration (for example, if an IPMI
boot preference is set).

This change removes the priority array, and replaces it with the
ipmi_bootdev data (and a persistent flag). We update the
default-conflict-resolution code to reflect the priorities between IPMI
and UUID preferences.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>