discover: Set OS Boot sensor with 0xa9

If we don't set all the things, we only get an assertion event, without
the state being asserted.

Despite the IPMI spec suggesting we only needed to set 0x10, aka the
assertion bits, testing reveals we want to set all of the things to
cause the BMC to assert the state.

Tested-by: Nick Bofferding <bofferdn@us.ibm.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>