drivers: modem: quectel-bg9x: mdm unsol ready

Using bg9x driver with bg95 specifically has the issue that BG95
modem replies with "RDY" when it's not fully initialized and AT
commands sent to the modem after this point would result in modem
reply of "ERROR". This commit adds a new prj.conf field in the build
configuration for custom unsolicited app ready string, with
the default being BG96 one ("RDY").

Signed-off-by: Andrei Hutanu <andrei.hutanu.i@gmail.com>

drivers/modem/Kconfig.quectel-bg9x

@@ -54,6 +54,14 @@ config MODEM_QUECTEL_BG9X_PASSWORD
 	  context.  This value is specific to the network provider and may
 	  need to be changed.
 
+config MODEM_QUECTEL_BG9X_UNSOL_RDY
+	string "Unsolicited response string expected from the modem"
+	default "RDY"
+	help
+	  This string is used to determine when modem powered and ready
+	  to accept AT commands. If this value is not matching the modem
+	  response, the init will fail with timeout.
+
 config MODEM_QUECTEL_BG9X_INIT_PRIORITY
 	int "quectel BG9X driver init priority"
 	default 80

drivers/modem/quectel-bg9x.c

@@ -921,7 +921,7 @@ static const struct modem_cmd response_cmds[] = {
 static const struct modem_cmd unsol_cmds[] = {
 	MODEM_CMD("+QIURC: \"recv\",",	   on_cmd_unsol_recv,  1U, ""),
 	MODEM_CMD("+QIURC: \"closed\",",   on_cmd_unsol_close, 1U, ""),
-	MODEM_CMD("RDY", on_cmd_unsol_rdy, 0U, ""),
+	MODEM_CMD(MDM_UNSOL_RDY, on_cmd_unsol_rdy, 0U, ""),
 };
 
 /* Commands sent to the modem to set it up at boot time. */

drivers/modem/quectel-bg9x.h

@@ -53,6 +53,7 @@
 #define MDM_APN_LENGTH			  32
 #define RSSI_TIMEOUT_SECS		  30
 
+#define MDM_UNSOL_RDY			CONFIG_MODEM_QUECTEL_BG9X_UNSOL_RDY
 #define MDM_APN				  CONFIG_MODEM_QUECTEL_BG9X_APN
 #define MDM_USERNAME			  CONFIG_MODEM_QUECTEL_BG9X_USERNAME
 #define MDM_PASSWORD			  CONFIG_MODEM_QUECTEL_BG9X_PASSWORD