Fix: Ensure correct target board is loaded when using Custom Board

[nvojnovic]

When selecting "Custom Board" in the project wizard with the target device EFR32ZG23B020F512IM48, the template defines RADIO_BOARD_EFR32ZG23, which causes radio_board_efr32zg23 to be loaded in target_boards.

This fix ensures that when a project is started with "Custom Board," the radio_no_board configuration is correctly loaded instead.