doc: board_porting: Fix Kconfig.defconfig example

The part that says "Always set CONFIG_BOARD here" is outdated. In HWMv2,
CONFIG_BOARD is set by the build system, and we don't want to encourage
users to set it manually.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>