Merge pull request #15148 from phlegx/mbed-os-5.15

Fix Ublox C030 SARA-R412M onboard cellular device serial (target default instance).

Author: 0xc0170

targets/TARGET_STM/TARGET_STM32F4/TARGET_STM32F437xG/TARGET_UBLOX_C030/ONBOARD_UBLOX.cpp

@@ -27,6 +27,7 @@ CellularDevice *CellularDevice::get_target_default_instance()
 {
 #if defined(TARGET_UBLOX_C030_R41XM)
     static UARTSerial serial(MDMTXD, MDMRXD, MBED_CONF_PLATFORM_DEFAULT_SERIAL_BAUD_RATE);
+    serial.set_flow_control(SerialBase::RTSCTS, MDMRTS, MDMCTS);
     static ONBOARD_UBLOX_AT device(&serial);
 #elif defined(TARGET_UBLOX_C030_N211)
     static UARTSerial serial(MDMTXD, MDMRXD, MBED_CONF_PLATFORM_DEFAULT_SERIAL_BAUD_RATE);