discover/platform-powerpc: read bootdev config from IPMI boot mailbox

discover/platform-powerpc: read bootdev config from IPMI boot mailbox

The IPMI Get System Boot Options commands includes parameter 7, the
"boot initiator mailbox". This can be used to hold arbitrary data to
influence the boot order.

Use this to provide an alternate bootdev configuration to Petitboot that
will override the one saved to NVRAM. This provides more fine grained
override options than the existing device-type based overrides.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>