FOC: Fix cache line size for Cortex A15

Added errata 774769, also there is no need to clean the cache on page table
update.
FOC: Disable the virtual console echo

This patch is needed for the Genode KDB terminal.

Fixes ssumpf/foc#1.
FOC: Fix 'user_flags' on x86

Use carry bit in EFLAGS to pass thread state to ex_reg caller. Do not touch
other bits, which might disable interrupts on kernel exit.

Fixes ssumpf/foc#2.
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.