Support inferences with multiple inputs and outputs

Build flatbuffers library.

Update network class to extract IFM and OFM dimensions from the tflite
file.

Update the uapi and core apis to support up to 16 IFM and OFM buffers
per inference.

Change-Id: I2f2f177aa4c2d5f9f50f23eb33c44e01ec2cbe09
diff --git a/kernel/ethosu_mailbox.h b/kernel/ethosu_mailbox.h
index c5865d0..5a77b54 100644
--- a/kernel/ethosu_mailbox.h
+++ b/kernel/ethosu_mailbox.h
@@ -100,8 +100,10 @@
  */
 int ethosu_mailbox_inference(struct ethosu_mailbox *mbox,
 			     void *user_arg,
-			     struct ethosu_buffer *ifm,
-			     struct ethosu_buffer *ofm,
+			     uint32_t ifm_count,
+			     struct ethosu_buffer **ifm,
+			     uint32_t ofm_count,
+			     struct ethosu_buffer **ofm,
 			     struct ethosu_buffer *network);
 
 #endif /* ETHOSU_MAILBOX_H */