soft_uart: make osmo_soft_uart_alloc() accept *cfg

Let the API user pass their own default config when allocating
a soft-UART.  Make the default config publicly accessible.

Change-Id: I7e78d60c747a8805064d5e4bacfd47a30bc65cba
Related: OS#4396
diff --git a/include/osmocom/core/soft_uart.h b/include/osmocom/core/soft_uart.h
index 71846ed..2b9d67f 100644
--- a/include/osmocom/core/soft_uart.h
+++ b/include/osmocom/core/soft_uart.h
@@ -72,9 +72,12 @@
 	void (*status_change_cb)(void *priv, unsigned int status);
 };
 
+extern const struct osmo_soft_uart_cfg osmo_soft_uart_default_cfg;
+
 struct osmo_soft_uart;
 
-struct osmo_soft_uart *osmo_soft_uart_alloc(void *ctx, const char *name);
+struct osmo_soft_uart *osmo_soft_uart_alloc(void *ctx, const char *name,
+					    const struct osmo_soft_uart_cfg *cfg);
 void osmo_soft_uart_free(struct osmo_soft_uart *suart);
 int osmo_soft_uart_configure(struct osmo_soft_uart *suart, const struct osmo_soft_uart_cfg *cfg);
 int osmo_soft_uart_set_rx(struct osmo_soft_uart *suart, bool enable);