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>

util/spkmodem_recv/Makefile

@@ -0,0 +1,9 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
+PREFIX  ?= /usr/local
+INSTALL ?= install
+
+spkmodem-recv:
+	$(CC) -o $@ $@.c
+install: spkmodem-recv
+	$(INSTALL) -d $(DESTDIR)$(PREFIX)/bin/
+	$(INSTALL) $< -t $(DESTDIR)$(PREFIX)/bin/