boards: arm: stm32f746g_disco: Use specified USB serial

The 'west flash' command allows specifying the port where the target is
attached via the '--serial' option, allowing users to set the USB serial
port for flashing. However, the 'stm32f746g_disco' script file currently
ignores the _ZEPHYR_BOARD_SERIAL variable set by this option, preventing
effective port specification.

This commit fixes it by correctly setting the openocd adapter serial
when _ZEPHYR_BOARD_SERIAL variable is set, enabling proper USB serial
port specification during flashing.

Signed-off-by: Gustavo Romero <gustavo.romero@linaro.org>