Replacing getchar with RawSerial getc in greentea-client.

This change prevents the standard library from allocating a large buffer
on the heap. On GCC_ARM, this is a saving of 1K. On ARM, this is a saving
of 64 bytes.

Author: bridadan

features/frameworks/greentea-client/source/test_env.cpp

@@ -533,7 +533,7 @@ enum Token {
  *
  */
 static int _get_char() {
-    return getchar();
+    return greentea_serial->getc();
 }
 
 /**