AP_HAL: added set_size_best() for ByteBuffer

libraries/AP_HAL/utility/RingBuffer.cpp

@@ -41,6 +41,20 @@ bool ByteBuffer::set_size(uint32_t _size)
     return true;
 }
 
+/*
+  set buffer size, accepting a smaller size if desired size isn't achievable
+ */
+bool ByteBuffer::set_size_best(uint32_t _size)
+{
+    while (_size > 0) {
+        if (set_size(_size)) {
+            return true;
+        }
+        _size = (3 * _size)/4;
+    }
+    return false;
+}
+
 uint32_t ByteBuffer::available(void) const
 {
     /* use a copy on stack to avoid race conditions of @tail being updated by

libraries/AP_HAL/utility/RingBuffer.h

@@ -52,6 +52,9 @@ public:
     // set size of ringbuffer, caller responsible for locking
     bool set_size(uint32_t size);
 
+    // set size of ringbuffer, reducing down if size can't be achieved
+    bool set_size_best(uint32_t size);
+    
     // advance the read pointer (discarding bytes)
     bool advance(uint32_t n);