board/stm32f769-disco: replace usage of default in-tree defconfig for kernel with custom one. ...

This allows to then get rid of the linux fragments and makes it possible to run 'make linux-menuconfig' and save the result with 'make linux-update-defconfig'.

This is not possible when using fragment files.

Initial commit for stm32f769-disco: ...

* Buildroot version 2021.02.7.
* Das U-Boot version 2021.10.
* Linux kernel version 5.15.3.
* Busybox 1.33.1.