scripts/functions: fix date munging when ns are not available

On some systems (eg. *BSD and Darwin), date does not support nanoseconds
(%N) precision. Instead of printing '%N' in this case, it just prints 'N'.
Fix the sed expression to handle this case.

 # m68k specific configuration file

 # depends on EXPERIMENTAL

 config ARCH_m68k

     select ARCH_SUPPORTS_32

     select ARCH_DEFAULT_32

     select ARCH_DEFAULT_BE

     select ARCH_SUPPORT_CPU

      help

       The m68k architecture