Standard L2 Genesis: Prevent start block misconfiguration

[maurelian]

From @tynes comment:

Noticed something here: dcdf2b7/op-node/rollup/types.go#L43

The op-node starts deriving on the L1 block after what is in its config. This means we need SystemConfig.startBlock() - 1 rather than the start block itself. Two options on how to handle this:

1. Have the genesis generation script fill in start block - 1 into the rollup config
2. Make the starting L1 block be optional in the rollup config and use the startBlock() value as the starting point if its not present

I lean towards the second, it removes config overhead.

[maurelian]

Also:

I think the second is doable so long as op-node can load it from the latest L1 block reliably. Otherwise it doesn't know which block the system config first existed in. In most cases (probably not all) op-node walks along L1 from the start block forwards loading data from the block its up to so it has a consistent view. This would need to be a case where it always goes to the latest value. The start block has to be immutable so that seems reasonable to me.