util/spkmodem_recv: Import from coreboot

Imported from util/spkmodem_recv at coreboot
revision:

e70bc423f9a2e1d13827f2703efe1f9c72549f20

This is a client for spkmodem, to allow serial
console via PC speaker.

I've decided to import it in lbmk, because I
heavily modified it. The patches will be
applied next.

Signed-off-by: Leah Rowe <leah@libreboot.org>