serial: Emulate break using control characters

Currently the magic SysRq functions are accessed by sending a break.
Unfortunately some networked serial proxies makes is difficult to send
a break meaning SysRq functions cannot be reached. We avoid this problem
by allowing the (fairly unlikely) sequence of ^B^R^K characters to
emulate a real break.

This approach is very nearly as robust as normal sysrq/break handling
because all trigger recognition happens during interrupt handling however
to emulate a break we must enter the ISR four times (instead of twice) and
manage an extra byte of state.

Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>

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