soft_uart: implement modem status lines and flow control

Change-Id: I26b93ce76f2f6b6fbf017f2684312007db3c6d48
Related: OS#4396
diff --git a/src/core/libosmocore.map b/src/core/libosmocore.map
index 4ae5108..fc81650 100644
--- a/src/core/libosmocore.map
+++ b/src/core/libosmocore.map
@@ -450,7 +450,9 @@
 osmo_soft_uart_set_tx;
 osmo_soft_uart_rx_ubits;
 osmo_soft_uart_tx_ubits;
+osmo_soft_uart_get_status;
 osmo_soft_uart_set_status;
+osmo_soft_uart_set_status_line;
 osmo_soft_uart_flush_rx;
 osmo_stat_item_dec;
 osmo_stat_item_flush;