subsys: kconfig: Remove 'default n' properties and clean up a bit

Bool symbols implicitly default to 'n'.

A 'default n' can make sense e.g. in a Kconfig.defconfig file, if you
want to override a 'default y' on the base definition of the symbol. It
isn't used like that on any of these symbols though.

Remove some 'default ""' properties on string symbols too.

Also make definitions more consistent by converting some

  config FOO
  	<type>
  	prompt "foo"

definitions to a shorter form:

  config FOO
  	<type> "foo"

This shorthand works for int/hex/string symbols too, not just for bool
symbols.

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