RingBuffer: Add ::clear() method to discard the buffer content

The same could be achieved with ::set_size(::get_size()), but was not
obvious and hurts code readability.

libraries/AP_HAL/utility/RingBuffer.cpp

@@ -36,6 +36,11 @@ uint32_t ByteBuffer::available(void) const
     return ((head > (_tail=tail))? (size - head) + _tail: _tail - head);
 }
 
+void ByteBuffer::clear(void)
+{
+    head = tail = 0;
+}
+
 uint32_t ByteBuffer::space(void) const
 {
     uint32_t _head = head;

libraries/AP_HAL/utility/RingBuffer.h

@@ -35,6 +35,9 @@ public:
     // number of bytes available to be read
     uint32_t available(void) const;
 
+    // Discards the buffer content, emptying it.
+    void clear(void);
+
     // number of bytes space available to write
     uint32_t space(void) const;