I was about to summarize the explanation, but the solution for jetpack-5.1.2., which allows to flash a custom board with custom pinmux, padvoltage, dtb and kernel does not work for jetpack 6.0 DP.
With jetpack 6.0 DP, only the step flashing a custom board as a devkit works (after replacing t186ref by generic in the command given in Post #2), but trying to install another dtb by using a conf file for the custom board currently installs the devkit’s dtb, not the custom board’s dtb.
My attempt for custom_board.conf is currently
source "${LDK_DIR}/jetson-orin-nano-devkit.conf";
PINMUX_CONFIG="Orin-custom_board-pinmux.dtsi";
PMC_CONFIG="Orin-custom_board-padvoltage-default.dtsi";
DTB_FILE="tegra234-custom_board.dtb";
TBCDTB_FILE="${DTB_FILE}";
Using that file works perfectly to flash my custom board with jetpack-5.1.2 (by replacing
jetson-orin-nano-devkit by custom_board in Post #2), but not with Jetpack 6.0 DP.