fastboot: make the raid autodetect code wait for all devices to init

The raid autodetect code really needs to have all devices probed before
it can detect raid arrays; not doing so would give rather messy situations
where arrays would get detected as degraded while they shouldn't be etc.

This is in preparation of removing the "wait for everything to init"
code that makes everyone pay, not just raid users.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>