cmake: fix <keys> argument in import_kconfig()

The signature of import_kconfig() take two mandatory arguments and one
optional:
> import_kconfig(<prefix> <kconfig_fragment> [<keys>])

but has been implemented in such a way that it loops all arguments after
the two mandatory args and sets the same list on those.

Fix this error by only setting the created variables on the third and
optional argument if it exists.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>