types: shorten boot_status definitions

struct boot_status is a bit misnamed; we report status on things that
aren't just the boot status (eg, discovery).

This change refactors struct boot_status into just struct status. We
give the type enum a name, and shorten the enum values to suit.

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