Kconfig: Clean up some symbol definitions

- Remove redundant 'n' defaults. 'n' is the default value for bool
  symbols.

  This makes the auto-generated documentation clearer as well: You get
  "implicitly defaults to n" instead of
  "- n if <propagated dependencies>".

- Shorten

      <type>
      prompt "foo"

  to

      <type> "foo"

  This works for all types, not just bool.

- Various formatting nits.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>