Update setup.sh to allow creating user repo for new custom shield

[petejohanson]

Currently, user configs allow having custom shields in them, thanks to being a "zephyr board root", but the setup.sh script only allows creating a new user config repo based on an in-tree shield.

I would expect the workflow something like

• Run setup.sh
• select an existing board
• select "Custom" when prompted for a shield
• be promoted for split or non-split
• be promoted for display name, e.g "Corne"
• be promoted for the shield identifier, with default inferred from display name, e.g. "corne" (note: can use tr to make lowercase from display name, and to translate spaces to underscores)
• once collected, would generate a new out of tree shield with reasonable defaults, and the usual west + GitHub actions configuration.