discover/ipmi: Increase IPMI timeout to a reasonable value

We've seen some IPMI timeouts during testing - meaning that an IPMI
bootdev setting will be ignored. This can result in a machine booting
from an incorrect boot device, or missing a 'safe mode' indication, or
incorrectly proceeding past petitboot.

The firmware & kernel has its own timeout & retry mechanism, so we
already have a little error-recovery there; the timeout in petitboot is
to prevent an indefinite block if the kernel interface isn't behaving
correctly.

So, this change bumps the timeout to a value that suits BMCs we've seen
in the field (specifying a 2 second timeout, with one retry).

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