Set BeaconRoot to something more meaningfull

[tzdybal]

          should we use `prevStateRoot` instead of a constant genesisHash. This field is supposed to be used to store any vital consensus layer information.

Originally posted by @gupadhyaya in evstack/go-execution-evm#30 (comment)

[rose2221]

would like to work on it! Could you assign it to me?

[tac0turtle]

amazing thank you