RPC library deadlock and/or vring memory corruption on Giga R1 (possibly other boards too).

[iabdalkader]

The attached sketch generates a random block of data on each core, and sends it to the other one to hash it. It then reads back the hash and compares it to the one generated locally.
If you run the sketch you'll see that it deadlocks and/or corrupts the memory in less than a minute or so. The Red LED blink means one side received a bad md5 checksum.

NOTE: you must fix the endpoint buffer size first before testing this, as in PR #801

rpc_md5sum.zip

[iabdalkader]

Fixed in #814