Synchronize host and target for nc serial auto test

We're ensuring target and host start-up sync here in 2 ways:
1) adding a delay on host side to make sure the serial
initialization can happen before sending a character is sent to target
2) in case of serial_nc_rx_auto.py test, we're sending a
first character S which will trigger the move from rx+tx
to NC+rx.

This should  avoid any crossing case due to HSOT being faster than target or vice-versa

libraries/tests/mbed/serial_nc_rx/main.cpp

@@ -11,22 +11,28 @@ int main() {
 
     char c = pc->getc();
 
-    delete pc;
 
-    // This should be true
-    if (c == 'E') {
-      Serial *pc = new Serial(USBTX, NC);
+    // This should be true, sync the start of test
+    if (c == 'S') {
+        pc->printf("RX OK - Start NC test\r\n");
 
-      pc->printf("RX OK - Expected\r\n");
+        // disconnect TX and  get char
+        delete pc;
+        pc = new Serial(NC, USBRX);
+        c = pc->getc();
+        if (c == 'E') {
+            // ok disconnect Rx and answer to host
+            delete pc;
+            pc = new Serial(USBTX, NC);
+            pc->printf("RX OK - Expected\r\n");
 
-      c = pc->getc();
-
-      // This should be false/not get here
-      if (c == 'U') {
-        pc->printf("RX OK - Unexpected\r\n");
-      }
-
-      delete pc;
+            c = pc->getc();
+            // This should be false/not get here
+            if (c == 'U') {
+                pc->printf("RX OK - Unexpected\r\n");
+            }
+        }
+        delete pc;
     }
 
     while (1) {

workspace_tools/host_tests/serial_nc_rx_auto.py

@@ -25,6 +25,29 @@ class SerialNCRXTest():
 
     def test(self, selftest):
         selftest.mbed.flush();
+        # Wait 0.5 seconds to ensure mbed is listening
+        time.sleep(0.5)
+
+        #handshake with target to sync test start
+        selftest.mbed.serial_write("S");
+
+        strip_chars = string.whitespace + "\0"
+
+        out_str = selftest.mbed.serial_readline()
+
+        if not out_str:
+            selftest.notify("HOST: No output detected")
+            return selftest.RESULT_IO_SERIAL
+
+        out_str_stripped = out_str.strip(strip_chars)
+
+        if out_str_stripped != "RX OK - Start NC test":
+            selftest.notify("HOST: Unexpected output. Expected 'RX OK - Expected' but received '%s'" % out_str_stripped)
+            return selftest.RESULT_FAILURE
+
+        # Wait 0.5 seconds to ensure mbed is listening
+        time.sleep(0.5)
+
         selftest.mbed.serial_write("E");
 
         strip_chars = string.whitespace + "\0"

workspace_tools/host_tests/serial_nc_tx_auto.py

@@ -25,6 +25,9 @@ class SerialNCTXTest():
 
     def test(self, selftest):
         selftest.mbed.flush();
+        # Wait 0.5 seconds to ensure mbed is listening
+        time.sleep(0.5)
+
         selftest.mbed.serial_write("S");
 
         strip_chars = string.whitespace + "\0"