Seu sistema não está congelando. Você acabou de direcionar ALL a sua saída para o console serial.
Passando argumentos do console para o kernel, você precisa incluir tanto o console local quanto a interface serial.
Veja TLDP - Remote Serial Console HOWTO - Capítulo 5. Configurar o kernel do Linux
Figure 5-2. Recommended kernel parameters, PCs with video card
console=tty0 console=ttyS0,9600n8
Kernel messages will appear on both the first virtual terminal and the serial port. Messages from the init system and the system logger will appear only on the first serial port. This can be slightly confusing when looking at the attached monitor: the machine will appear to boot and then hang. Don't panic, the init system has started but is now printing messages to the serial port but is printing nothing to the screen. If a getty has been configured then a login: prompt will eventually appear on the attached monitor.
For PCs without a video card, this HOWTO suggests the kernel parameters:
Figure 5-3. Recommended kernel parameters, PCs without video card
console=ttyS0,9600n8
These parameters are passed to the booting kernel by the boot loader. Next we will configure the boot loader used by your Linux installation to pass the console parameters to the kernel.