Add a magic OPAL log command

[ajdlinux]

It would be really handy to have a magic command to get data from the OPAL log.

In particular, given how slow getmem is, it would be nice to have an easy way to fetch the last X bytes of the log, which would involve reading the memcons struct and calculating the right offset into the log.

[shenki]

I had just knocked together a rough version of this. I will create a better UI it first, as it was a bit ghetto.