FOC: Fetch serial input without '-serial_esc' option

With this patch, the kernel fetches serial data also when the
'-serial_esc' option is not set.

To keep the patch minimally invasive, the 'serial ESC' functionality gets
enabled by default and the existence of the '-serial_esc' argument gets only
checked when an ESC character was received and the decision of entering the
kernel debugger or not needs to be made.

Fixes ssumpf/foc#3.