[nrf noup] Clean Kconfig.defaults to support "Thread and Wi-Fi" builds

1. Group Kconfig options into categories to make it easier
   to see discrepancies between Thread and Wi-Fi and help
   choose the right value when both transports are enabled.

   That is, prefer:

   config A
     default X if CHIP_WIFI
     default Y if NET_L2_OPENTHREAD

   over:

   if CHIP_WIFI
   config A
     default X
   endif

   if NET_L2_OPENTHREAD
   config A
     default Y
   endif
2. Remove redundant defaults that are already selected in
   other Kconfig files.

Signed-off-by: Damian Krolik <damian.krolik@nordicsemi.no>