core: Add software UART implementation

This patch brings a Work-in-Progress implementation of the software
UART (Universal Asynchronous Receiver/Transmitter) to libosmocore.

Not only it will be useful in the context of retro-networking, but
also it's needed for the MS-side CSD implementation (see OS#4396).

It should be noted that the definition of struct osmo_soft_uart
is intentionally kept private, since the API is not stable yet.

Currently, the following limitations apply:

* Only the receiver part is implemented, the transmitter is TBD.
* Parity checking is not implemented in the receiver part.
* Software flow control is not implemented.

These missing components will be addressed in subsequent patches.

Change-Id: I2ca95963fd5852ddb89bdd35b86b31489127fe84
diff --git a/src/core/Makefile.am b/src/core/Makefile.am
index 326c68d..2efebd8 100644
--- a/src/core/Makefile.am
+++ b/src/core/Makefile.am
@@ -63,6 +63,7 @@
 	signal.c \
 	sockaddr_str.c \
 	socket.c \
+	soft_uart.c \
 	stat_item.c \
 	stats.c \
 	stats_statsd.c \