After installing Jetpack and flashing L4T R21.4 my Jetson board does not work anymore. I also tried using -S 12 GiB.
Flashing seems to be correct, but after the first reboot the boot process stops at
tegra_dvfs: vdd_gpu connected to regulator
After resetting the board, I can only see
U-Boot SPL 2014.10-rc2-00002-g105d2f3 (Jun 25 2015 - 15:19:50)
in the serial console and it does not even start to load the kernel.
The serial console log of the flash (with -S 12 GiB) and the first boot can be found here: