AP_HAL: resolved -Woverloaded-virtual warning

the function:
virtual size_t AP_HAL::Print::write(const uint8_t *buffer, size_t size);

was hidden in all derived classes by their
virtual size_t write(uint8_t) = 0; implementations.

To solve this, a non-virtual write(const uint8_t *, size_t) that calls a
virtual write_implementation was added.

This isn't necessary atm, because the derived classes don't call
write(const uint8_t *, size_t), BUT this decreases the apm2-quad Program
size by 40 bytes :D and removes warnings.

libraries/AP_HAL/utility/Print.cpp

@@ -30,7 +30,7 @@
 using namespace AP_HAL;
 
 /* default implementation: may be overridden */
-size_t Print::write(const uint8_t *buffer, size_t size)
+size_t Print::write_implementation(const uint8_t *buffer, size_t size)
 {
   size_t n = 0;
   while (size--) {

libraries/AP_HAL/utility/Print.h

@@ -47,8 +47,9 @@ class AP_HAL::Print {
     virtual size_t write(uint8_t) = 0;
 
     size_t write(const char *str) { return write((const uint8_t *)str, strlen(str)); }
-    virtual size_t write(const uint8_t *buffer, size_t size);
-    
+    size_t write(const uint8_t *buffer, size_t size) {return write_implementation(buffer, size);}
+    virtual size_t write_implementation(const uint8_t *buffer, size_t size);
+  public:
     size_t print(const char[]);
     size_t print(char);
     size_t print(unsigned char, int = DEC);