fix compiler warnings - board.c - strncpy, allow trailing null (5th character)

[nerdCopter]

• board.c strncpy allow trailing null (5th character)
• confirmed this is how Betaflight does it too
• fixes:

./src/main/pg/board.c:47:9: warning: 'strncpy' output truncated before terminating nul copying 4 bytes from a string of the same length [-Wstringop-truncation]
   47 |         strncpy(boardConfig->manufacturerId, TARGET_MANUFACTURER_IDENTIFIER, MAX_MANUFACTURER_ID_LENGTH);

[nerdCopter]

tested several FC found zero ill-effects.