[coreboot] spkmodem

[Vladimir 'φ-coder/phcoder' Serbinenko]

Hello, all. I send a patch to make coreboot output console using system
beeper and a corresponding code to interpret those beeps. I publish it
under GPLv2+ but other licensing arrangements are possible. This was
very useful to get console on X201 where I have neither serial nor EHCI
debug.