|Autobooting Linux from U-Boot|
The bootdelay variable defines a delay, in seconds, before running the autoboot command defined by bootcmd. During the bootdelay countdown, you can interrupt the autobooting by pressing any key. This will let you enter the U-Boot command line interface.
The bootcmd variable defines a command executed by U-Boot automatically after the bootdelay countdown is over. Typically, this would be run netboot to boot Linux from TFTP during development or run flashboot to boot Linux from the NOR Flash on deployed units. The latter is how bootcmd is set on the modules at the factory.
In deployed configurations, where boot time to the service provided by your embedded device is critical, you will probably want to set bootdelay to 0:
STM32F4X9-SOM> setenv bootdelay 0
This will make sure that on each power on / reset U-Boot immediately executes the command defined by bootcmd, typically booting Linux from the on-module Flash.
With bootdelay set to 0 the U-Boot countdown is disabled, so there is a question how you enter the U-Boot command monitor, should you need that for some reason. To do so, push the Ctrl-C keys down on the serial console and don't release them until you have hit the reset button on the baseboard. This will interrupt the U-Boot bootcmd sequence and let you enter the U-Boot command monitor:
U-Boot 2010.03-cortexm-2.0.0 (May 5 2016 - 20:31:28)