Get X86 boards compliant with default configuration guidelines

[erwango]

Created from #7151, dedicated to X86 boards

Following introduction of "default board configuration guidelines" (#6858), one part of the work is to move boards already ported to zephyr to conform with guidelines available here:
http://docs.zephyrproject.org/porting/board_porting.html#default-board-configuration

https://github.com/zephyrproject-rtos/zephyr/tree/master/boards/arm/nucleo_f429zi could be used as reference for this update.

[erwango]

Following creation of #19116 that aims at instantiating a systematic test to check compliance, I'm closing this issue.