Enable USART3 support for STM32F412 and use it as the default NUCLEO-F412ZG nsh console path.
The ST-LINK virtual COM port on this board is wired to USART3 on PD8/PD9. Restore USART2 to its header-pin routing, switch the nsh defconfig to USART3, and update the board documentation to match the shipped solder-bridge configuration described in issue #17722.
Signed-off-by: aviralgarg05 <gargaviral99@gmail.com>
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