PS_ON configurable boot state

Allows the user to select wheter or not the PSU should be turned on or
kept in standby when marlin boots
Merge pull request #630 from dumle29/Marlin_v1

PS_ON configurable boot state
Made instructions more clear.

Not sure what happened to my last commit so this includes that one + reordering the steps.
Merge pull request #632 from dbenamy/Marlin_v1

Made instructions more clear.