kconfig: replace a `switch()' statement by a more flexible `if()' statement

With the upcoming dynamical configuration prefix, we can no longer assume that
the prefix will start by a 'C'. As such, we can no longer hardcode this value in
the `case ...:', so replace the `switch() { ... }' statement by a more flexible
'if () { ... }' statement.

Signed-off-by: Arnaud Lacombe <lacombar@gmail.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Michal Marek <mmarek@suse.cz>