applications: serial_lte_modem: Add workaround for upmerge

Next upmerge brings Kconfig changes that we might want
to cherry-pick earlier so prepare a workaround for
allowing build against pre-upmerge or cherry-pick tree.

Signed-off-by: Seppo Takalo <[email protected]>

Author: SeppoTakalo

applications/serial_lte_modem/overlay-ppp-cmux-linux.conf

@@ -10,7 +10,6 @@ CONFIG_SLM_PPP=y
 CONFIG_SLM_CR_TERMINATION=y
 
 CONFIG_MODEM_CMUX_MTU=127
-CONFIG_MODEM_CMUX_WORK_BUFFER_SIZE=536
 CONFIG_SLM_CMUX_UART_BUFFER_SIZE=600
 
 # Enable SLM UART backend

applications/serial_lte_modem/src/slm_cmux.c

@@ -31,6 +31,11 @@ LOG_MODULE_REGISTER(slm_cmux, CONFIG_SLM_LOG_LEVEL);
 #define DLCI_TO_INDEX(dlci) ((dlci) - 1)
 #define INDEX_TO_DLCI(index) ((index) + 1)
 
+/* Workaround before upmerge */
+#ifndef MODEM_CMUX_WORK_BUFFER_SIZE
+#define MODEM_CMUX_WORK_BUFFER_SIZE CONFIG_MODEM_CMUX_WORK_BUFFER_SIZE
+#endif
+
 static struct {
 	/* UART backend */
 	struct modem_pipe *uart_pipe;
@@ -42,8 +47,8 @@ static struct {
 
 	/* CMUX */
 	struct modem_cmux instance;
-	uint8_t cmux_receive_buf[CONFIG_MODEM_CMUX_WORK_BUFFER_SIZE];
-	uint8_t cmux_transmit_buf[CONFIG_MODEM_CMUX_WORK_BUFFER_SIZE];
+	uint8_t cmux_receive_buf[MODEM_CMUX_WORK_BUFFER_SIZE];
+	uint8_t cmux_transmit_buf[MODEM_CMUX_WORK_BUFFER_SIZE];
 
 	/* CMUX channels (Data Link Connection Identifier); index = address - 1 */
 	struct cmux_dlci {