Add an option to change the default shell.

[meator]

This adds an option to set the default shell of the root user and documents the live.shell kernel option.

I know that a similar change has already been closed (#99). My change makes this more configurable and optional. And it documents the live.shell option that isn't documented anywhere else and is pretty useful.

I'll understand if you close this, but I consider this helpful for people who are customizing their iso.

[Vaelatern]

I do not object to this.

Configuration in this particular area might be useful. The previous PR posited "The default shell should be bash" which was wrong. This one posits "The default shell should be configurable on image creation" which seems reasonable.

I'm pretty sure the commit message is incorrect.

[meator]

I've made the commit message clearer. I've also added a check that increases STEP_COUNT by one when the -S option is used.