discover: separate status-reporting function from boot() callback

discover: separate status-reporting function from boot() callback

Currently, the device_discover_boot_status function is both used for
internal status updates, as well as the callback passed to boot().

This change splits this into two functions; one for the latter and one
for the former. The latter just has a void * for its first argument, to
match the boot_status_fn type.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>